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GENERAL  BOTANY 


FOR 


UNIVERSITIES  AND  COLLEGES 


BY 

HIRAM  D.  DENSMORE,  M.A. 

\v 

PROFESSOB  OF   BOTANY   AT   BELOIT   COLLEGE,  BELOIT,  WISCONSIN 


WITH  ORIGINAL  ILLUSTRATIONS  BY 

THE  AUTHOR  AND  BY  M.  LOUISE  SAWYER,  M.S.,  FORMERLY  INSTRUCTOR 
IN  BOTANY  AT  BELOIT  COLLEGE 


GINN  AND  COMPANY 

BOSTON     •     NEW   YORK     •     CHICAGO     •     LONDON 
ATLANTA     •     DALLAS     •     COLUMBUS     •     SAN   FRANCISCO 


BIOLOGY 

LIBRARY 

G 


ENTERED  AT  STATIONERS'  HALL 


COPYRIGHT,   1920,   BY 
HIRAM  D.  DENSMORE 


ALL   RIGHTS   RESERVED 
320.10 


HEPT,  lUrfc. 


GINN  AND  COMPANY  •  PRO- 
PRIETORS •  BOSTON  •  U.S.A. 


PREFACE 

This  textbook  is  an  outgrowth  of  the  author's  long  experience 
in  giving  to  college  students  an  introductory  course  in  botany. 
It  has  been  used  by  the  writer,  in  a  briefer  typewritten  form, 
with  several  classes  at  Beloit  College  and  by  botanical  instruc- 
tors in  three  other  institutions.  It  has  thus  been  adapted  to  the 
needs  of  students  as  a  result  of  practical  experience  and  criticism. 

The  author's  aim  in  writing  the  book  has  been  to  furnish  the 
student  with  clear  statements,  properly  related,  of  the  essen- 
tial biological  facts  and  principles  which  should  be  included 
in  a  first  course  in  college  botany  or  plant  biology.  It  has  been 
assumed  that  the  text  would  be  supplemented  by  lectures  and 
readings  to  adapt  it  to  particular  needs  in  different  institutions. 
Such  topics  as  relate  to  the  economic  aspects  of  botany  have 
consequently  been  treated  concisely,  with  the  idea  that  they 
would  be  elaborated  and  extended  variously  by  different  instruc- 
tors. In  all  cases,  however,  the  biological  principles  underlying 
such  practical  aspects  of  botany  have  been  supplied  in  the  text. 
The  author  hopes,  therefore,  that  the  book  will  furnish  both 
students  and  instructors  with  a  helpful,  connected  statement  of 
the  more  important  facts  and  principles  of  modern  botany. 

Content  and  use.  The  subject  matter  of  the  text  is  divided 
into  three  parts,  which  are  so  arranged  that  they  may  be  used 
together,  as  the  basis  for  a  year's  course  in  general  botany,  or 
separately,  for  courses  of  one  term  or  one  semester  in  length. 

Part  I  is  intended  to  present  the  biological  aspects  of  plant 
life  from  the  standpoint  of  structure  and  function,  on  the  basis 
of  studies  of  the  higher  and  more  familiar  seed  plants.  Three 
main  themes  are  considered  in  this  part  of  the  text,  namely,  the 
relations  and  adjustments  of  the  higher  plants  to  other  organisms 

v 

498431 


vi  GENERAL  BOTANY 

and  to  the  inanimate  forces  and  materials  of  their  environment ; 
the  cellular  structure  of  plants  and  its  relation  to  growth,  repro- 
duction, and  the  anatomy  of  woody  and  herbaceous  plants ;  the 
phenomena  of  reproduction,  and  their  relation  to  crossing,  hybridi- 
zation, and  plant  breeding.  The  author  has  used  this  portion 
of  the  text,  with  some  selections  from  Part  II,  as  a  basis  for  a 
semester  course  in  plant  biology. 

Parts  I  and  II  could,  with  equal  advantage,  be  combined  as 
the  basis  for  a  course  on  the  higher  plants  during  the  latter 
portion  of  the  college  year. 

Part  II  deals  with  the  morphology,  life  histories,  and  evolution 
of  the  main  plant  groups.  In  the  chapter  devoted  to  the  fungi 
emphasis  is  placed  upon  the  nature  of  enzymes  and  fermenta- 
tion and  upon  the  relations  of  these  processes  to  parasitism, 
disease,  and  decay.  This  chapter  also  gives  an  introduction  to 
the  important  aspects  of  fungi  related  to  plant  diseases  and  plant 
pathology.  In  the  treatment  of  the  higher  spore-bearing  and 
seed  plants  the  author  has  given  nearly  equal  attention  to  the 
evolution  of  structure  and  to  reproduction,  instead  of  placing 
the  main  stress  upon  reproductive  features,  as  is  often  done  in 
elementary  textbooks.  In  the  parts  relating  to  structure  the 
teachings  of  the  newer  anatomy  are  followed.  This  method  of 
treatment  has  been  found  to  be  simpler  pedagogically,  and  more 
in  accord  with  modern  botanical  knowledge,  than  the  usual 
presentation  based  upon  the  older  anatomy. 

Part  III  is  intended  to  serve  as  an  introduction,  to  field  work 
and  to  the  study  of  the  interesting  biological  and  economic 
aspects  of  a  few  important  families  and  species  represented  in 
the  spring  flora.  To  this  end  considerable  space  is  allotted  to  the 
study  of  trees  and  their  importance  to  man.  The  main  problems 
of  forestry  are  emphasized  concretely  in  connection  with  the 
life  of  a  few  selected  species  of  forest  trees.  The  herbaceous 
species  of  the  monocotyledons  and  the  dicotyledons  are  also 
treated  from  their  biological  and  economic  aspects,  in  order  to 
indicate,  if  possible,  the  point  of  view  from  which  additional 
species  may  be  studied. 


PREFACE  vii 

A  brief  chapter  on  plant  associations  is  appended  to  the 
studies  of  families  and  species,  in  order  to  stimulate  thought 
and  observation  along  these  lines.  No  attempt  has  been  made 
to  present  plant  ecology  in  a  formal  manner,  since  the  entire 
treatment  of  families  and  species  in  Part  III  is  ecological  in  its 
nature  and  thus  presents  the  elements  of  ecology  in  a  concrete 
mariner  in  connection  with  field  studies. 

Part  III  is  therefore  not  primarily  taxonomic,  but  rather 
biological,  economic,  and  ecological  in  its  point  of  view,  and  is 
thus  in  harmony  with  the  treatment  of  plants  in  the  first  two 
parts  of  the  text. 

Distinctive  features.  Plants  are  presented  throughout  the 
text  as  living,  active  organisms,  comparable  to  animals  and  with 
similar  general  physiological  life  functions. 

The  purely  scientific  and  descriptive  portions  of  the  text  are 
directly  linked  with  the  theoretical  and  economic  aspects  of 
biology  which  are  of  immediate  human  interest.  Thus,  the 
cellular  structure  of  organisms  is  directly  related  to  growth, 
>to  the  structure  and  life  of  trees,  and  to  the  economic  value  of 
wood ;  the  chapter  on  vegetative  and  sexual  reproduction  is 
followed  by  a  presentation  of  the  essential  facts  and  theories 
relating  to  hybridization,  breeding,  and  evolution.  The  topic 
of  evolution,  however,  is  not  presented  as  a  theory  by  itself, 
but  rather  as  nature's  method  of  plant  breeding  and  improve- 
ment, closely  related  to  similar  processes  carried  on  by  man. 

The  chapters  on  plant  physiology  are  summarized  and  closely 
correlated  with  the  seasonal  life  of  such  common  plants  as  the 
bean,  clover,  and  locust.  Physiological  processes  are  thus  made 
directly  and  concretely  applicable  to  the  seasonal  life  of  well- 
known  plant  species.  More  space  is  devoted  to  the  cell,  mitosis, 
and  cell  structure  than  is  usually  accorded  such  topics  in  an 
elementary  textbook. 

Two  reasons  seem  to  the  author  to  justify  giving  this  larger 
space  to  cellular  biology.  The  first  is  derived  from  experience 
in  presenting  these  subjects  to  beginning  students,  who  usually 
manifest  more  interest  in  them  than  in  most  other  aspects  of 


viii  GENERAL  BOTANY 

botany.  The  second  relates  to  the  increasing  importance  of  first- 
hand knowledge  of  the  cell  and  mitosis  to  a  proper  understand- 
ing of  the  modern  literature  and  popular  discussions  concerning 
genetics,  heredity,  and  breeding.  This  part  of  biology  is  also  of 
fundamental  importance  in  psychology,  physiology,  and  sociology. 
In  case  teachers  do  not  care  to  give  much  time  to  laboratory  work 
on  mitosis,  the  outline  and  figures  of  the  text  should  furnish  a 
valuable  basis  for  class  discussion  of  this  important  topic. 

The  presentation  of  plant  structure  from  the  viewpoint  of 
modern  anatomy  is  also  new  in  an  elementary  textbook,  but 
such  treatment  is  justified  by  the  author's  experience  in  teach- 
ing this  aspect  of  the  subject  to  beginning  students  in  botany. 
It  is  hoped  also  that  the  outline  figures  and  the  simple  treat- 
ment in  the  text  will  enable  instructors  not  familiar  with  this 
phase  of  botany  to  present  the  subject  from  the  standpoint  of 
modern  plant  anatomy. 

The  laboratory  directions,  which  are  printed  under  separate 
cover,  are  intended  to  stimulate  interest  and  observation  without 
giving  detailed  guidance  in  laboratory  work. 

Acknowledgments.  In  closing  the  author  desires  to  acknowl- 
edge his  indebtedness  to  former  students  and  assistants,  as  well 
as  to  other  botanists  and  friends,  who  have  materially  aided  him 
in  the  completion  of  this  book. 

First  acknowledgements  are  due  to  Miss  M.  Louise  Sawyer, 
instructor  in  botany  at  Knox  College  and  a  former  student  and 
instructor  at  Beloit  College.  Miss  Sawyer's  criticisms  and  sug- 
gestions were  invaluable  in  the  early  stages  of  writing  the  text 
and  while  it  was  being  tested  with  classes  in  Beloit  College. 
In  the  making  and  reproduction  of  the  original  drawings  Miss 
Sawyer  deserves  equal  credit  with  the  author  for  whatever  of 
originality  or  helpfulness  they  may  be  found  to  possess.  She 
reproduced  all  of  the  original  drawings  in  ink  and  made  the 
camera  drawings  on  the  cell,  mitosis,  and  anatomy  from  slides 
in  the  author's  laboratory.  The  drawings  on  the  pollen  tubes 
and  sperm  atogenesis  in  Iris  are  taken  from  an  original  paper 
published  by  Miss  Sawyer. 


PREFACE  ix 

Special  acknowledgment  is  also  made  for  the  privilege  of 
using  in  this  text  various  illustrations  from  Bergen's  "  Founda- 
tions of  Botany,"  Bergen  and  Caldwell's  "  Introduction  to 
Botany "  and  "  Practical  Botany,"  and  Bergen  and  Davis's 
"  Principles  of  Botany." 

Acknowledgments  are  due  to  Dr.  H.  S.  Conard  of  Grinnell 
College  for  the  critical  reading  of  the  preliminary  manuscript 
of  Parts  I  and  II  and  for  criticisms  and  suggestions  based 
upon  the  use  of  the  text,  in  typewritten  form,  with  his  own 
classes. 

The  author  is  also  indebted  to  Dr.  E.  C.  Jeffrey  of  Harvard 
University  for  some  of  the  microphotographs  of  woody  and 
herbaceous  stems  used  in  the  text,  and  to  Dr.  W.  J.  V.  Osterhout 
of  Harvard  for  valuable  suggestions. 


CONTENTS 


PART  I.    BIOLOGY  OF  THE  HIGHER  SEED  PLANTS 
SECTION  I.    PLANTS  AND  THE  ENVIRONMENT 

CHAPTER  PAGE 

I.  THE  RELATIONS  OF  PLANTS  TO  THE  ENVIRONMENT  .     .     .         3 
II.  THE   FORM  AND   ADJUSTMENTS   OF   THE   PLANT  BODY  TO 

THE  ENVIRONMENT       .............       14 

SECTION  II.    CELL  STRUCTURE  AND  ANATOMY 

III.  THE  CELLULAR  STRUCTURE  OF  PLANTS    .......       45 

IV.  HISTORICAL  SKETCH  (TiiE  CELL  AND  THE  CELL  THEORY)      54 
V.  GROWTH  AND  CELL  DIVISION  .....     ......       60 

VI.  THE  STRUCTURE  AND  FUNCTIONS  OF  STEMS,  ROOTS,  AND 

LEAVES       ..........     .......       83 

SECTION  III.    PHYSIOLOGY 

VII.  NUTRITION  AND  SEASONAL  LIFE  OF  PLANTS     .....     117 
VIII.  THE  RELATION  OF  PLANTS  TO  WATER      .     .     .....     137 

SECTION  IV.    REPRODUCTION 

IX.  VEGETATIVE  AND  SEXUAL  REPRODUCTION    .....     .  154 

X.  PLANT  BREEDING  AND  EVOLUTION  .........  174 

XI.  HISTORICAL  DEVELOPMENT  OF  BOTANY  AND  THE  BIOLOGI- 

CAL SCIENCES  ................  212 

PART  II.   THE  PLANT  GROUPS 


XII.  THE  ALG^     .     .     ...     .     .     .     .........  219 

XIII.  THE  FUNGI     .......     ,     .     .     .......  242 

XIV.  BRYOPHYTES  (LIVERWORTS  AND  MOSSES)      ......  287 

XV.  PTERIDOPHYTES  (FERNS,  EQUISETA,  AND  CLUB  MOSSES)    .  297 

XVI.  GYMNOSPERMS    ................  321 

XVII.  ANGIOSPERMS  (DICOTYLEDONS)     .....     .     .     .     .     .  337 

xi 


xii  GENERAL  BOTANY 

PART  III.  REPRESENTATIVE  FAMILIES  AND  SPECIES  OF  THE 
SPRING  FLORA 

CHAPTER 

XVIII.  DESCRIPTIVE  TERMS 353 

XIX.  TREES,  SHRUBS,  AND  FORESTS 3gg 

XX.  HERBACEOUS  AND  WOODY  DICOTYLEDONS   .  396 

XXI.  MONOCOTYLEDONS       .........  416 

XXII.  PLANT  ASSOCIATIONS      .     .     ....:*•.  435 

INDEX     . 447 


GENERAL  BOTANY 

PART  I.    BIOLOGY  OF  THE  HIGHER 
SEED  PLANTS 


SECTION  I.    PLANTS  AND  THE 
ENVIRONMENT 

CHAPTER  I 

THE  RELATIONS  OF  PLANTS  TO  THE  ENVIRONMENT 

Plant  biology,  or  elementary  botany,  is  chiefly  concerned  with 
the  structure  and  activities  of  plants  and  with  their  relations  to 
the  living  and  lifeless  objects  and  forces  which  surround  them 
and  constitute  their  environment.  It  is  evidently  impossible  at 
the  outset  of  our  study  to  present  more  than  a  general  survey 
of  the  relations  and  functions  of  plants  in  nature,  but  it  is 
hoped  that  such  a  survey  will  stimulate  the  interest  of  the 
student  in  the  larger  aspects  of  botany  and  will  serve  to  give 
him  a  new  appreciation  of  the  great  importance  of  plants  in 
the  world  of  living  things. 

In  order  to  present  this  more  comprehensive  idea  of  plant 
life  clearly  and  concisely  we  shall  need  to  consider  first  the 
peculiar  relations  of  the  world  of  vegetation  to  inanimate 
nature,  including  the  soil,  the  air,  and  such  forces  of  its  en- 
vironment as  light  and  gravity.  This  discussion  can  then  be 
logically  followed  by  considering  the  relation  which  plants 
sustain  to  animate  nature,  including  man  and  other  animals. 
With  this  brief  introduction  we  may  proceed  directly  to  the 
consideration  of  these  topics. 

THE  INANIMATE  ENVIRONMENT 

The  forces.  Light  and  gravity,  which  impinge  upon  the 
bodies  of  plants  from  without,  are  used  by  the  higher  plants 
in  such  a  way  as  to  secure  the  advantageous  placing  or  ad- 
justment of  their  leaves,  roots,  stems,  and  flowers  in  the  soil 

3 


GENERAL   BOTANY 


and  the  air.  Fig.  1,  a,  illustrates  this  placing  of  the  organs 
of  a  young,  growing  plant  in  the  positions  usually  assumed 
by  them,  and  we  shall  find  that  these  positions  are,  in  part  at 
least,  directly  traceable  to  the  effects  of  environmental  forces 
acting  on  the  plant  body.  We  may  note  in  the  figure  that 
the  main  taproot  grows  vertically  downward  toward  the  center 

of  the  earth,  but 
that  the  lateral, 
or  secondary,  roots 
assume  quite  a 
different  position, 
growing  at  more 
or  less  definite  an- 
gles from  the  pri- 
mary taproot.  The 
main  stein  likewise 
grows  in  a  direc- 
tion exactly  oppo- 
site to  that  of  the 
main  root,  as  it 
should  do  in  order 
to  perform  its  as- 
signed task  of  dis- 


FIG.  1.   Two  seedlings  of  the  scarlet-runner  bean 

a,  a  seedljng  growing  in  a  normal  position  with  its  organs 
properly  adjusted  to  light  and  soil ;  b,  the  stem  (hypocotyl) 
has  adjusted  itself  hy  curvature  in  response  to  gravity  act- 
ing as  a  stimulus 


playing  the  leaves 
to  the  sunlight 
and  the  flowers  to 
wind  and  insects. 
The  student  has  doubtless  noticed  also  that  the  leaves  of  plants 
may  assume  the  normal  horizontal  position  illustrated  in  the 
figure,  or,  in  the  case  of  plants  before  a  window,  the  leaf  blades 
may  take  an  oblique  position,  so  that  they  face  the  maximum 
light.  If  the  plant  represented  in  Fig.  1,  a,  had  been  grown 
in  a  flowerpot  laid  on  its  side,  as  in  6,  or  if  it  had  started  its 
growth  on  a  steep  hillside,  the  roots,  stem,  and  leaves  would 
have  turned  so  as  finally  to  assume  the  most  favorable  relation 
to  sunlight,  air,  and  soil.  Botanists  have  demonstrated  the  fact, 
which  we  shall  explain  more  in  detail  later,  that  growing  roots, 


RELATION  OF  PLANTS   TO  ENVIRONMENT  5 

stems,  and  leaves  are  sensitive  to  such  forces  as  light  and  grav 
ity,  and  that  they  have  the  ability  to  respond  to  them  by  definite 
growth  movements  which  are  dependent  upon  the  nature  of 
the  organ  concerned.  This  sensitiveness  and  response  enables 
the  organs  of  plants  to  adjust  themselves  to  their  environment 
by  placing  themselves  in  proper  positions  for  the  absorption  of 
foods  and  energy.  For  example,  the  root  system  secures  a  wide 
distribution  in  the  soil,  since  its  primary  and  lateral  roots  are 
able  to  respond  to  the  gravity  stimulus  by  growth  in  different 
directions  with  reference  to  the  stimulating  force.  Similarly, 
the  main  stem  and  its  branches  respond  to  gravity  and  light, 
and  leaves  are  able  to  adjust  themselves  to  light  coming  from 
different  angles. 

Plants  are,  therefore,  sensitive  organisms  which  sustain  a  defi- 
nite relation  to  the  forces  of  their  environment,  and  they  are 
able  to  use  these  forces  in  assuming  the  various  attitudes  and 
positions  which  are  most  advantageous  to  them  in  a  given 
place  and  time. 

The  energy  of  the  sunlight  is  also  absorbed  and  transformed 
into  heat  and  manufacturing  energy,  to  be  used  in  the  making 
of  sugar,  starch,  and  cellulose  by  the  green  portions  of  plants. 

The  materials.  Green  plants  are  likewise  able  to  use  the 
material  substances  of  the  air  and  the  soil  as  no  other  organ- 
isms can  do,  and  thereby  to  build  up  food  for  themselves  and  for 
the  animal  world,  for  we  shall  find  that  all  animals  are  ultimately 
dependent  upon  plants  for  sustenance  and  continued  existence. 

This  peculiar  relation  of  green  plants  to  the  inorganic  sub- 
stances of  their  surroundings  is  easily  understood  from  the 
diagrammatic  drawing  (Fig.  2). 

The  root  system  of  the  plant  represented  in  Fig.  1,  a,  has  been 
admirably  distributed  through  a  wide  area  of  soil  by  means  of 
the  sensitiveness  of  the  roots  to  gravity.  From  this  soil  the 
roots  absorb  water  (composed  of  hydrogen  and  oxygen  (H2O)) 
and  soil  salts  containing  the  various  substances,  such  as 
nitrogen,  sulphur,  phosphorus,  and  potash,  which  the  plant 
uses  in  building  its  own  foods  and  its  living  substance.  The 
leaves  take  in  carbon  dioxide  (CO2)  from  the  air,  and  practically 


6  GENERAL  BOTANY 

all  parts  of  the  plant  body  may  absorb  oxygen.  These  life- 
less substances  of  its  environment  are  therefore  continually 
streaming  into  the  plant  from  its  surroundings  through  its 
roots,  stems,  and  leaves.  The  striking  and  characteristic  thing 
about  the  green  plant  is  that  it  can  take  these  simple,  life- 
less substances  of  the  air  and  the  soil  and  construct  foot!  from 
them  both  for  itself  and  for  animals,  and  that,  like  animals,  it 
can  build  these  inert  foods  into  living  substance,  endowed  with 
the  unique  and  characteristic  properties  of  life.  Within  recent 
times  chemists  have  been  able,  in  the  chemical  laboratory,  to 
compound  such  food  substances  as  sugar  and  even  simple  forms 
of  nitrogenous  substances  or  proteins;  but  no  chemist  has  yet 
been  able  to  do  what  the  plant  does  daily,  namely,  to  convert 
lifeless  matter  into  living  matter.  The  power  of  green  plants 
to  make  organic  compounds  is  due  to  the  fact  that  they  have 
a  green  substance,  chlorophyll,  within  their  tissues  which  enables 
them  to  absorb  and  use  the  energy  of  light  in  \the  making  of 
starch  and  sugar.  When  the  streams  of  carbon  dioxide  pour 
into  the  leaf  from  the  air,  they  meet  there  the  water  derived  by 
the  roots  from  the  soil.  In  the  presence  of  the  green  pigment, 
chlorophyll,  the  living  substance  of  the  leaf  is  then  able  to 
utilize  the  outside  energy  of  the  sun  and  unite  the  carbon  of 
the  inflowing  carbon  dioxide  (CO2)  with  the  hydrogen  and 
oxygen  of  the  water  (H2O)  to  make  sugar  and  starch.  The 
sugar  is  then  combined  with  the  nitrogen,  sulphur,  and  phos- 
phorus, derived  from  soil  salts  which  are  absorbed  by  the  roots, 
into  nitrogenous  foods  and  ultimately  into  the  living  substance 
of  the  plant  body. 

A  part  of  these  foods,  or,  more  probably,  the  living  sub- 
stance itself,  is  then  broken  down  by  union  with  the  oxygen 
which  enters  the  plant  at  various  points  in  the  plant  body  and 
brings  about  oxidation,  or  respiration,  as  in  our  own  bodies.  This 
oxidation  process  resembles  somewhat  the  oxidation  which  occurs 
when  coal  is  burned  in  a  furnace  or  in  a  stove,  and  results  in 
the  formation  of  cell  energy  and  certain  waste  products  in  the 
plant  comparable  to  the  heat,  gases,  and  ash  produced  by  the 
combustion  of  coal  and  wood  in  a  stove.  This  waste  material 


RELATION  OF  PLANTS   TO  ENVIRONMENT  7 

is  often  cast  out  of  the  plant  in  much  the  same  form  in  which 
it  came  into  it,  namely,  carbon  dioxide  and  water ;  but  more  fre- 
quently these  substances  are  used  over  again  to  rebuild  living 
substance  and  plant  food.  The  energy  released  by  oxidation  is 
used  by  the  plant  in  its  vital  processes  of  growth,  reproduction, 
and  repair. 

The  peculiar  relation  which  nonliving  matter  in  the  form  of 
foods  sustains  to  the  living  matter  composing  the  bodies  of 
animals  and  plants  may  be  expressed  in  various  ways.  Cuvier 
and  Huxley  put  this  relation  in  a  striking  manner  by  comparing 
living  beings  to  a  whirlpool. 

The  whirlpool  is  permanent,  but  the  particles  of  water  which 
constitute  it  are  constantly  changing.  Those  which  enter  it  on  the 
one  side  are  whirled  around  and  temporarily  constitute  a  part  of 
its  individuality ;  and  as  these  leave  it  on  the  other  side  their 
places  are  made  good  by  newcomers. 

It  is  undoubtedly  true  that  the  green  plant  illustrates  the 
whirlpool  conception  more  correctly  than  any  other  organism, 
since  it  alone  of  all  living  forms  has  the  power  of  converting 
the  simple  lifeless  compounds  of  the  earth  and  air  into  living 
matter.  When  we  study  the  next  topic,  we  shall  see  that  animals 
and  man  are  not  so  intimately  related  to  inorganic  nature  as 
green  plants  are. 

THE  ANIMATE  ENVIRONMENT 

Most  plants  are  intimately  associated  with  other  plants  and 
also  with  certain  forms  of  animal  life  which  affect  their  lives 
in  various  ways  and  so  constitute  a  part  of  their  natural 
environment. 

Thus,  a  plant  on  a  lawn,  in  a  meadow,  or  in  a  forest  is  closely 
surrounded  by  its  neighbors,  which  compete  with  it  for  light, 
air,  and  soil  space  in  which  to  expand  and  obtain  food.  Tall 
plants  shade  shorter  ones,  and  closely  matted  plants,  like  grass, 
are  likely  to  occupy  the  soil  space  to  the  exclusion  of  less 
social  plants.  In  a  forest  the  competition  of  trees  is  always 
evidenced  by  the  death  of  those  which  fail  to  keep  pace  with 


8 


GENERAL  BOTANY 


PLANT  BODY 


INCOME 


neighbors  in  the  upward  growth  toward  the  light.  A 
tree  developing  in  the  open  always  has  a  more  symmetrical 
form  and  a  fuller  leafage  than  one  of  the  same  species  growing 
in  a  forest.  In  these  and  various  other  ways  the  environment  of 
any  given  individual  or  species  of  plant  is  vitally  affected  by 
neighboring  plants  of  the  same  community  or  society. 

Animals  are  also  factors  in  the  plant's  environment  in  that 
they  tend  to  injure  or  destroy  plant  life  for  food  or  for  protec- 

tion. Some  plants, 
like  the  cacti  and 
thistles,  are  armed 
and  thus  protected 
against  the  higher 
forms  of  animals  ; 
but  even  these  ex- 
ceptional forms  are 
subject  to  attack 
by  smaller  forms, 
which  may  infest 
the  roots  or  bore 
into  the  stem  or 
leaves  and  either 


1.  ENERGY 


2.  INORGANIC 
MATERIALS 


d.   OXYGEN 


FIG.  2.    Diagram  illustrating  the  nutrition  of 
a  green  plant 


At  the  extreme  left  the  income  in  energy  and  inorganic 
materials  is  shown  ;  within  the  circle  the  main  organic 
substances  constructed  within  the  plant  body  are  in- 
dicated. At  the  extreme  right  the  outgo  in  energy  and 
inorganic  materials  is  indicated  ;  within  the  circle  the 
energy  and  waste  products  of  respiration  (destructive 
process)  are  shown 


kill  or  injure  them. 
So,  while  the 
plant  draws  upon 
its  inanimate  envi- 
ronment for  energy 

and  foods,  it  rmist  needs  compete,  in  a  hard  struggle  for  its 
existence,  with  the  other  living  organisms,  including  both  plants 
and  animals,  by  which  it  is  surrounded.  These  living  forms 
constitute  its  animate  environment. 

RELATIONS  OF  PLANTS  TO  ANIMALS 

Plants  help  man  and  other  animals  by  giving  them  food 
and  protection  and  by  creating  an  environment  favorable  to 
their  needs  and  comfort.  Plants  are  also  of  the  greatest  indus- 
trial and  commercial  importance  on  account  of  the  food,  shelter, 


RELATION  OF  PLANTS  TO  ENVIRONMENT 


9 


OUTGO 


2.  ORGANIC 
FOODS 


and  energy  which  they  furnish  for  the  maintenance  and  welfare 
of  man  in  the  home  and  in  the  industries. 

Food  relations  of  plants  and  animals.  The  food  relations  of 
plants  to  man  and  animals  can  be  most  easily  understood  by  com- 
paring the.  income  and  outgo  of  a  common  green  plant  with  that 
of  a  man,  or  of  an  animal  similar  to  man  in  its  feeding  habits. 

For  this  comparison  Figs.  2  and  8  may  be  used.  The  cir- 
cle in  each  figure  may  be  taken  to  represent  the  plant  or 
animal  body  con- 
taining the  living  INCOME 
substance  that  both 
does  the  feeding  and 
produces  the  energy 
for  the  living  or- 
ganisms. These  fig- 
ures illustrate  the 
fact  that  the  ani- 
mal bears  the  same 
general  relation  as 
the  plant  to  lifeless 
matter,  to  the  pro- 
duction of  energy, 
and  to  the  elimina- 
tion of  wastes.  Life- 
less matter,  in  the 

form  of  foods,  —  bread,  butter,  and  meat,  —  is  taken  into  the 
body,  digested  and  assimilated  by  the  living  substance  of  the 
animal  body,  and  converted  temporarily  into  living  matter  as 
illustrated  in  the  figure.  The  living  matter  thus  produced 
from  lifeless  foods  is  then  oxidized,  and  this  process  yields 
energy  for  the  ordinary  animal  activities.  The  waste  materials 
produced  by  this  oxidation  are  then  cast  out  of  the  body  in 
the  form  of  carbon  dioxide,  water,  and  nitrogenous  wastes ; 
and  there  is  some  loss,  of  energy  with  the  heat  that  escapes 
from  the  body. 

A  real  difference  between  the  animal  and  the  plant,  however, 
lies  in  the  nature  of  the  lifeless  matter  taken  in.  In  the  animal 


3.  INORGANIC 
MATERIALS 


a.  WATER 

b.  OXYGEN 

C.  SALTS 


FIG.  3.    Diagram  illustrating  the  nutrition  of 
an  animal 

The  income,  outgo,  and  products  of  constructive  and 

destructive  processes  are  shown  as  for  the  green  plant 

in  Fig.  2.   Compare  Figs.  2  and  3  and  consult  the  text 

for  further  discussion 


10  GENERAL  BOTANY 

the  bread,  butter,  and  meat  (which  are  taken  as  representative 
of  animal  foods)  are  real  organic  food  materials,  which  have 
been  previously  built  up  from  raw,  inorganic  materials  by 
green  plants. 

Thus,  bread,  as  we  all  know,  is  made  of  flour,  which  con- 
sists of  finely  ground  kernels  of  wheat.  The  starch  and  the 
smaller  quantities  of  nitrogenous  gluten  of  which  flour  is 
largely  composed  were  built  up,  as  we  have  already  learned, 
in  the  leaves  of  wheat  plants  from  carbon  dioxide,  water, 
and  soil  salts,  absorbed  from  the  air  and  the  soil.  After 
being  constructed  in  the  leaves,  they  were  passed  into  the 
wheat  kernels  as  reserve  food  for  the  growth  of  the  embryos 
in  the  seed  when  the  seed  should  germinate.  This  ready- 
made  food  of  the  green  plant  is  seized  upon  by  animals  for 
their  maintenance.  In  a  similar  manner  the  meat  which  con- 
stitutes the  food  of  some  animals  is  ultimately  traceable  to  a 
plant  origin,  since  even  carnivorous  animals  (meat  eaters)  derive 
their  food  from  herbivorous  animals  (plant  eaters).  For  example, 
cats  live  on  mice,  and  the  mice  get  their  food  from  grain  or 
other  plant  parts ;  and  cattle,  pigs,  chickens,  and  fish,  which 
are  the  most  common  sources  of  meat  for  man's  use,  depend 
upon  grass,  hay,  grain,  and  water  plants  for  their  food  supply. 
All  of  these  animals  would  starve  in  a  comparatively  short  time 
without  the  continued  supply  of  food  which  green  plants  build 
from  carbon  dioxide,  water,  and  soil  salts. 

This  general  relation  of  the  lifeless  world  of  matter  and 
force  to  the  living  world  of  animals  and  plants  is  illustrated 
diagrammatically  in  Fig.  4.  It  is  only  necessary  to  add  to 
the  explanation  already  given  the  well-known  fact  that  the 
death  of  animals  and  man  is  frequently  caused  by  colorless 
plants  known  as  bacteria.  Entire  crops  are  also  often  de- 
stroyed by  parasitic  fungi  such  as  the  rusts  of  grains,  and  all 
decay  is  induced  by  these  colorless  fungous  plants.  This  decay 
finally  converts  the  bodies  of  the  dead  animals  and  plants 
into  gases  and  "other  chemical  substances,  which  filter  into  the 
soil  and  form  a  part  of  the  lifeless  matter  which  is  absorbed 
as  raw  food  material  by  green  plants.  The  most  familiar 


RELATION  OF  PLANTS  TO  ENVIRONMENT         11 


instance  of  this  fact  is  that  of  the  farmer  fertilizing  his  fields 
with  fertilizers  composed  of  the  remains  of  plants  and  the 
excrements  of  animals. 

We  have  now  completed  the  general  survey  of  the  relation 
of  green    and   colorless  plants  to  that   part  of   nature  which 

INCOME  OUTGO 


I.  FROM  SUN 

ENI 


GREEN    PLANT 


FIG.  4.    Organic  food  cycle 

What  three  classes  of  organisms  are  represented  as  concerned  with  the  construction 

and  use  of  foods  in  the  world  ?  What  part  does  each  play  in  the  maintenance  and 

consumption  of  its  food  supply  ?  What  distinctive  relation  has  the  green  plant  to  the 

maintenance  of  the  world's  food  supply? 

constitutes  their  environment.  The  details  of  the  picture  will 
be  filled  in  as  we  proceed  in  later  chapters  with  the  structure 
and  work  of  plants. 

Plants  and  the  environment  of  animals.  We  are  not  accus- 
tomed to  think  of  the  earth  as  it  would  be  without  the  plant 
life  which  both  clothes  and  beautifies  it.  The  protective,  en- 
vironmental function  of  plants  is  perfectly  obvious,  however, 
without  lengthy  comment  or  discussion.  The  great  forests  form 
a  necessary  environment  for  hundreds  of  species  of  animals, 
ranging  from  the  larger  forms,  which  make  their  homes  in 


12  GENEKAL  BOTANY 

the  trees,  to  the  smaller  and  humbler  forms,  like  insects, 
which  inhabit  the  crevices  of  decaying  logs  and  bark  and 
the  leaf  mold  created  by  the  decay  of  fallen  leaves.  Winter 
to  these  creatures  of  the  forest  is  less  severe  than  it  would  be 
in  an  open  and  barren  region,  and  summer  brings  an  abundance 
of  food,  with  protection  from  heat.  Similarly,  an  intimate  knowl- 
edge of  the  life  of  our  common  birds,  insects,  fishes,  and  mam- 
mals would  reveal  the  all-important  influence  of  plants  on  the 
environment  of  these  animals  in  the  meadows,  lakes,  and  streams 
which  they  inhabit.  The  great  deserts,  likewise,  are  habitable 
by  a  limited  number  of  animal  forms,  including  man,  on  account 
of  the  protection  and  food  supplied  by  the  cacti  and  other  desert 
plants  which  have  become  adapted  to  such  arid  regions. 

On  account  of  these  environmental  relations  of  plants  to 
man,  the  culture  of  certain  kinds  of  plants  which  are  useful 
for  food  and  for  lumber  and  for  ornamental  purposes  has 
assumed  a  larger  and  larger  place  in.  industry  and  commerce 
as  civilization  has  advanced. 

Industrial  and  commercial  relations  of  plants.  The  great 
importance  of  modern  industrial  botany  has  led  to  the  estab- 
lishment, by  our  national  and  state  governments,  of  important 
departments  for  the  investigation  and  culture  of  plants.  In 
these  departments  hundreds  of  expert  botanists  are  studying 
those  aspects  of  plants  and  plant  culture  which  are  deemed  to 
be  of  the  greatest  importance  to  man. 

Agriculture,  and  its  relation  to  plants,  is  one  of  the  great 
lines  of  endeavor  connected  with  practical  botany.  Forestry, 
although  not  yet  so  fully  developed  in  our  own  country,  bids 
fair  to  receive  an  increasing  amount  of  attention  as  our  forests 
become  depleted  and  the  supply  of  lumber  decreases. 

Plant  breeding,  or  the  production  of  better  kinds  of  fruits, 
grains,  and  ornamental  plants,  is  receiving  wide  attention  and 
interest  at  the  present  time.  As  a  result  of  this  great  activ- 
ity on  the  part  of  individuals  and  of  our  national  and  state 
governments,  many  improved  kinds  of  food  and  forage  plants 
have  already  been  bred,  and  the  future  promises  even  greater 
improvements  in  both  agricultural  and  horticultural  varieties. 


RELATION  OF  PLANTS  TO  ENVIRONMENT          13 

Plant  breeding,  aside  from  its  great  practical  importance,  is 
of  equal  theoretical  interest  to  students  of  evolution  and 
genetics,  since  the  practical  breeding  of  plants  is  based  upon 
theoretical  principles  which  form  the  basis  for  generalizations 
in  these  subjects. 

The  study  of  plant  diseases,  or  plant  pathology,  is  also  of  the 
greatest  practical  interest  to  the  growers  of  all  kinds  of  plants. 
It  has  been  found  that  plants  are  not  only  subject  to  diseases 
produced  by  such  colorless  plants  as  rusts  and  smuts  of  grain 
and  tree-killing  fungi  but  that  they  are  also  subject  to  bac- 
terial diseases,  as  is  the  case  with  man  and  other  animals.  To 
eradicate  these  diseases  and  save  the  great  losses  produced 
by  them  annually,  our  government  has  now  organized  a  very 
large  department  of  plant  pathology.  It  is  not  necessary  to 
point  out  in  greater  detail  the  importance  of  plants  in  nature 
and  the  consequent  interest  and  importance  attached  to  the 
study  of  plant  biology  and  botany. 

SUMMARY 

1.  Plants  use  the  forces  of  their  environment  —  light,  heat,  and 
gravity  —  in  adjusting  their  organs  properly,  in  the  air  and  in  the 
soil,  for  the  absorption  of  raw  food  materials. 

2.  Green  plants  are  unique  among  living  objects  in  being  able 
to  build  foods  for  themselves  and  for  animals  out  of  simple  chemical 
substances  which  occur  in  the  air  and  soil. 

3.  Green  plants  are  therefore  intermediates  between  inorganic, 
lifeless  nature  and  the  more  highly  organized  animals. 

4.  Plants  are  also  important  to  animals  and  man  in  forming  a 
proper  environment  for  their  protection  and  welfare. 

5.  Plants  are  of  the  greatest  importance  to  man  industrially  and 
commercially. 


CHAPTER  II 

THE  FORM  AND  ADJUSTMENTS  OF  THE  PLANT  BODY 
TO  THE  ENVIRONMENT 

We  learned  in  the  previous  chapter  that  the  organs  of  the 
plant  body  —  roots,  stem,  leaves,  and  flowers  —  were  so  arranged 
and  adjusted  to  the  materials  and  forces  of  the  environment 
as  to  secure  abundant  food  and  energy  for  the  maintenance  of 
life.  This  necessary  arrangement  and  adjustment  of  plant  organs 
to  their  surroundings  is  secured  in  part  by  an  inherited  plan  of 
architecture,  which  governs  the  formation  and  growth  of  organs 
in  the  embryo  and  in  the  adult  organisms,  and  in  part  also 
by  powers  of  movement,  called  tropisms,  by  which  growing 
plant  organs  place  themselves  in  proper  relations  to  soil,  light, 
and  air. 

THE  FORM  AND  PLAN  OF  THE  PLANT  BODY 

If  we  observe  the  plant  body  of  most  plants,  we  shall  see 
that  it  consists  of  a  main  stem  and  root,  which  constitutes 
its  central  axis,  and  of  lateral  organs  in  the  form  of  leaves, 
flowers,  branches,  and  lateral  roots.  More  careful  observa- 
tion of  such  a  plant  will  also  show  that  the  lateral  organs 
are  not  placed  on  the  main  axis  in  an  indefinite  manner,  but 
that  they  have  a  definite  order  and  arrangement  inherited  from 
a  long  line  of  plant  ancestors.  It  will  soon  become  evident 
also  that  by  this  inherited  plan  of  the  plant  body  the  organs 
are  so  related  to  each  other  and  to  air,  light,  and  soil  as  to 
make  the  plant  as  a  whole  a  good  working  organism  in  its 
efforts  to  secure  food  for  sustenance  and  growth.  These  facts 
will  become  more  and  more  obvious  as  we  proceed  to  study 
the  relations  of  leaves,  branches,  and  secondary  roots  to  each 
other  and  to  the  main  axis. 

14 


THE  PLANT  BODY 


15 


Blade 


FIG.  5.    Body  plan  of  the  lilac  and  apple 

a,  lilac  twig  with  cyclic  leaf  arrangement ;  b,  apple 

twig  with  spiral  leaf  arrangement.  Compare  Fig.  5, « 

and  b,  with  Fig.  7,  a  and  b 


The,  leaves  of  plants 
(Fig.  5)  spring  from 
definite  points  on  the 
main  axis,  which  are 
called  nodes,  and  the 
nodes  are  separated  by 
definite  spaces,  or  in- 
tervals of  stem,  called 
internodes.  The  leaves 
also  grow  from  definite 
points  at  the  nodes  and 
are  so  arranged  on  adja- 
cent nodes  that  a  leaf  at 
one  node  never  stands 
immediately  above  the 

]eaf    at    t]ie    no(Je     ^-^ 
J 

below  it.    By  this  ar- 
rangement the  shading 

of  the  green  tissues  of  one  leaf  by  another  above  it  is  avoided. 

The  leaves  on  a  stem  are  therefore  arranged  in  mathematical 

order,  which  usually  conforms  to  one 

of  two  types  of  arrangement,  namely, 

the  cyclic  arrangement  and  the  spiral 

arrangement. 

In  the  cyclic  leaf  arrangement  two 

or  more  leaves  occur  at  each  node, 

and   the    leaves    of    adjacent    nodes 

alternate  with  each  other.     In  such 

plants  as  the   lilac    and  catnip  two 

leaves  are  placed  opposite  each  other 

at  each  node,   alternating   with  the     FlG  6    Leaf  arrangement  on 

pairs  of  leaves  at  the  nodes  immedi-       a  lilac  twig  seen  from  above 

ately    above    and   below    them    (FigS.       Note  the  alternating  leaf  pairs, 

5, '«,  and  6).    The  entire  leafage  of 

such  a  stem  is  thus  arranged  in  four 

vertical  rows  of  leaves  up  and  down  the  stem,  each  row  being 

separated  from  its  neighbor  by  an  angle  of  90°.    Each  leaf  in 


16 


GENERAL  BOTANY 


any  one  row  is  therefore  separated  from  tne  leaf  immediately 
above  or  below  it  in  the  same  row  by  two  internodes  of  the  stem. 
If  one  observes  a  stem  or  a  branch  from  above,  the  four  rows  of 
leaves  are  plainly  visible,  and  the  entire  upper  portion  of  the 
shoot  has  the  appearance  of  Fig.  6.  In  the  diagrammatic  draw- 
ings (Fig.  7),  in  which  the  leaves,  nodes,  and  internodes  are  repre- 
sented as  if  projected  on  a  plane,  the  circles  represent  nodes  and 
the  intervening  spaces  internodes.  The  shaded  central  portion 
is  meant  to  represent  the  small  leaves  and  very  short  internodes 

of  the  terminal 
bud.  Outside 
of  this  central 
shaded  area  the 
interned al  spaces 
widen  gradually 
toward  the  pe- 
riphery of  each 
figure  to  repre- 
sent the  gradual 
FIG.  7.  Diagrams  showing  cyclic  and  spiral  leaf  lengthening;  of 

arrangements 

the  internodes  on 

a,  cyclic  leaf  arrangement  of  the  lilac,  reduced  to  one  plane.  ,  ,, 

The  circles  represent  nodes,  and  the  spaces  between  inter-  a  s^em  trom  its 
nodes.  The  leaves  are  seen  to  be  in  four  longitudinal  rows  apex  to  i^S  base, 
along  the  stem.  6,  spiral  leaf  arrangement  of  the  apple,  ,p,  .  ,  , 

reduced  to  one  plane.    Nodes  and  internodes  as  in  a  InlS      lengthen- 

ing of  the  lower 

internodes  as  the  leaves  grow  in  surface  is  of  advantage  in 
preventing  shading  of  the  lower  leaves  by  large  leaves  above 
them.  The  cyclic  leaf  arrangement  (Fig.  7,  a)  is  thus  seen  to 
be  well  adapted  to  the  exposure  of  leaves  to  sunlight  without 
interference  or  shading  by  their  neighbors  on  the  same  shoot. 

In  the  spiral  leaf  arrangement  (Figs.  5,  £>,  and  8)  only  one  leaf 
is  placed  at  a  node,  and  the  leaves  spring  from  the  nodes  so  as 
to  effect  a  spiral  distribution  along  the  twigs  and  young  shoots. 
Since  any  given  leaf  is  always  separated  from  a  leaf  above  or 
below  it  in  the  same  straight  line  by  two  or  more  internodes 
(Fig.  7,  5),  according  to  the  type  of  spiral  arrangement  in  any 
particular  case,  the  same  advantages  as  regards  sun  exposure 


THE  PLANT  BODY 


17 


Axillary  flowering  branc/i 


are  secured  by  the  spiral  as  by  the  cyclic  leaf  arrangement.  The 
spiral  arrangement  of  leaves  is  often  distorted  by  growth  and  is 
therefore  difficult  to  trace  on  mature  shoots.  It  is  consequently 
more  clearly  evident  at 
the  apex  of  a  branch, 
where  the  twisting 
effects  of  growth  are 
less  evident. 

Since  the  buds  and 
lateral  branches  arise 
from  the  axils  of  the 
leaves  of  the  season, 
it  is  evident  that  the 
entire  branch  system 
must  follow  either  the 
spiral  or  the  cyclic 
plan  upon  which  the 
leaves  are  arranged 
(Fig.  8).  The  general 
form  and  leaf  exposure 
of  an  adult  plant  will 
therefore  be  largely  de- 
termined by  the  above 
body  plans  and  by  the 
manner  in  which  they 
are  worked  out  through 


Axillary  leafy  branch- 


-^Secondary  roots 


-Primary  taproot 


FIG.  8.   Body  plan  and  arrangement  of  organs  in 
the  buckwheat  (Fagopyrum) 

Note  that  the  branches  spring  from  the  angle  (axil 
of  the  leaf)  between  the  leaf  and  stem,  and  are  there- 
fore disposed  spirally  on  the  stem,  like  the  leaves. 
The  adjustment  of  leaves  and  roots  to  light  and  soil 
is  evident 


growth    as   the   plant 
matures. 

The  root  systems  of 
plants  are  less  definite 
in  their  arrangement 
than  the  branch  sys- 
tems, probably  owing  to  the  fact  that  their  soil  environment  is 
more  uniform  and  less  exacting  than  the  forces  and  elements  to 

o 

which  the  aerial  portions  of  the  plant  body  are  subjected.  Thus, 
there  may  or  there  may  not  be  a  primary  central  taproot  cor- 
responding to  the  central  stem  axis  aboveground.  The  secondary 


18 


GENERAL  BOTANY 


roots  usually  arise  in  an  orderly  succession,  but  there  are  no 
definite  nodes  and  internodes.  We  shall  learn  later  that  the 
successful  distribution  of  roots  in  the  soil  is  determined  in 
most  plants  more  largely  by  their  powers  of  adjustment  through 
growth  movements  than  by  their  arrangement  on  any  fixed  body 

plan.     The   relation    of    the 

body  plan  of  aerial  plant 
parts  to  the  ultimate  form 
of  some  common  herbaceous 
and  woody  plants  will  now 
be  considered. 

The  form  and  development 
of  herbaceous  plants.  In  a 
number  of  herbaceous  plants 
the  terminal  bud  grows  rap- 
idly during  the  early  part  of 
the  season  and  produces  a 
stout  central  stem,  while  the 
lateral  buds  remain  dormant 
and  thus  fail  to  produce  lat- 
eral branches  to  any  consid- 
erable extent.  The  leaves 
FIG.  9.  Body  plan  and  pyramidal  form  spring  from  the  nodes  either 
of  an  herbaceous  plant 


The  ultimate  form  of  this  cineraria  plant 
and  the  exposure  of  its  leaves  to  light  are  de- 
termined largely  by  its  spiral  body  plan,  the 
alternation  of  nodes  and  internodes,  and  the 
continued  growth  of  the  terminal  bud.  Pho- 
tograph by  Fuller,  from  Cowles's  "  Ecology  " 


in  spiral  or  in  cyclic  order, 
the  lower  leaves  being  usu- 
ally somewhat  larger  and  hav- 
ing longer  petioles  than  the 
upper  and  younger  ones.  The 
result  of  such  growth  is  a 
plant  of  pyramidal  form  (Fig.  9),  in  which  the  entire  leafage 
is  admirably  exposed  to  sunlight. 

The  root  system  of  such  plants  is  variously  arranged,  but 
without  the  definite  plan  of  the  aerial  parts,  owing  to  the  fact 
that  the  main  root  has  no  definite  nodes  from  which  lateral 
roots  spring.  Since,  however,  the  soil  salts  and  water  are  usu- 
ally distributed  equally  on  all  sides  of  the  r6ots,  and  since  there 
is  no  danger  from  shading,  it  is  evident  that  roots  do  not  need 


THE  PLANT  BODY  19 

the  more  accurate  plan  of  the  stem,  leaves,  and  branches.  In 
other  herbaceous  plants  the  lateral  buds  may  all  produce  lateral 
branches,  and  in  that  case  the  plant  assumes  a  symmetrical 
pyramidal  or  rhomboidal  form  like  the  ragweed  (Ambrosia), 
aster,  and  Russian  thistle.  All  gradations  in  symmetry  and  reg- 
ularity of  form  between  the  erect  and  the  pyramidal  or  rhom- 
boidal types  mentioned  occur  in  herbaceous  plants,  according  to 
the  relation  between  the  growth  of  the  main  axis  and  the  later 
branches.  Where  great  irregularity  in  the  growth  of  the  lateral 
branches  occurs,  the  body  plan  is  often  obscured  in  the  adult 
plant  but  is  evident  in  youth  and  in  the  arrangement  of  leaves 
and  buds  on  the  branches. 

The  form  and  development  of  trees.  Among  trees  there  is  the 
same  general  relation  between  the  body  plan,  the  growth  of 
buds  and  branches,  and  the  ultimate  form  of  the  tree  and  its 
adaptation  to  the  forces  and  materials  of  the  environment  as 
we  have  noted  above  in  herbaceous  plants. 

THE  ERECT  TKEE  TYPE 

The  pines.  In  erect  tree  types  like  the  common  pine  (Fig.  10) 
the  main  axis  terminates  each  season  in  a  twig  which  bears  the 
buds  for  the  next  season's  growth.  These  buds  consist  of  a  vig- 
orous terminal  bud,  which  continues  the  elongation  of  the  axis, 
and  of  from  three  to  five  vigorous  lateral  buds  clustered  at  the 
base  of  the  terminal  bud.  The  remaining  buds  of  the  terminal 
twig  are  latent  and  rarely  if  ever  grow  into  lateral  branches. 

When  spring  arrives  the  vigorous  apical  bud  elongates  and 
produces  the  terminal  twig  of  the  season  with  its  spirally 
arranged  scale  and  needle  leaves.  Meanwhile  the  vigorous  lat- 
eral buds  elongate  and  produce  a  whorl  of  branches  which  are 
separated  from  each  other  by  inconspicuous  internodes.  Since 
these  branches  arise  from  buds  which  are  really  arranged  spirally 
in  the  axils  of  minute  scale  leaves,  they  are  called  false  whorls, 
to  distinguish  them  from  true  whorls,  which  could  only  arise  on 
trees  with  a  cyclic  body  plan.  The  newest  false  whorl  of  each 
season  is  separated  from  the  false  whorl  of  the  previous  season, 


20 


GENERAL  BOTANY 


'alse  whorls 


immediately  below  it,  by  that  portion  of  the  terminal  twig  which 
bore  latent  buds  and  is  hence  devoid  of  branches  (Fig.  10,  «)• 
Since  but  one  false  whorl  of  branches  is  produced  each  season 
in  the  manner  described  above,  the  trunk  of  an  adult  pine  tree 
presents  a  series  of  false  whorls  of  branches  from  base  to  apex, 

separated  by  smooth 
portions  of  the  trunk 
(Fig.  10,  6).  These 
smooth,  branchless  por- 
tions of  the  trunk  are 
not  internodes,  as  they 
are  sometimes  thought 
to  be,  but  rather  rep- 
resent those  portions 
of  the  terminal  twigs 
of  each  season  where 
the  latent  buds  failed 
to  produce  branches. 
Since  the  branches  at 
the  apex  of  the  tree  are 
younger  and  so  shorter 
than  those  toward  the 
base,  the  result  is  a 
pyramidal  tree  with  a 
strong  central  excur- 
rent  trunk. 

The  needle  leaves 
and  reproductive  cones 
are  always  produced 
by  buds  at  the  ends  of  the  lateral  branches,  and  on  account  of 
their  spiral  arrangement  they  are  so  placed  as  to  be  admirably 
exposed  to  the  sunshine,  which  aids  the  leaves  in  their  food- 
making,  and  to  the  wind,  which  helps  to  scatter  the  winged 
seeds  of  the  pine  cones.  The  body  plan  and  the  method 
of  bud  growth,  therefore,  combine  in  the  pines  to  produce  a 
tree  of  great  beauty  and  of  nice  adaptation  to  environmental 
conditions. 


False 
whorl 


FIG.  10.   Pine  trees  illustrating  the  erect  type 

a,  a  young  pine ;  b,  a  mature  pine.  Observe  the  ex- 
current  trunk,  the  false  whorls  of  branches,  and  the 
pyramidal  form.  Consult  the  text  for  the  main  fac- 
tors concerned  in  the  seasonal  growth  (1-7)  and  the 
development  of  these  external  features  of  the  pine 


THE  PLANT  BODY 


21 


SPREADING  TREE  TYPES 

The  elm.  In  trees  of  the  spreading  type,  like  the  elm,  poplar, 
oak,  and  hickory,  the  same  general  plan  of  development  can  be 
traced  as  that  outlined  for  the  pine,  except  that  the  terminal 
bud  of  the  main 
axis  is  replaced 
by  a  lateral  bud 
after  a  few  years, 
so  that  no  main 
central  excurrent 
trunk  is  contin- 
ued throughout 
the  life  of  such 
trees.  In  the  elm 
the  terminal  bud  , 

is  replaced  each  I 

season  by  a  lat- 
eral bud,  which 
produces  a  main 
central  excurrent 
trunk  for  a  few 
years  (Fig.  11,  e). 
Ultimately,  as  in 
all  such  trees 
of  the  spreading 
type,  a  few  lateral 
branches  gain  the 


FIG.  11.  Growth  of  the  American  elm,  an  illustration  of 
the  spreading  type  of  trees 

ascendancy  and  The  letters  from  left  to  right  show  several  stages  in  the  de- 
form all  of  the  velopment  of  the  elm.  The  ultimate  form  is  determined  by 
body  plan,  the  method  of  bud  growth,  and  pruning  effects. 
The  corresponding  letters  on  each  figure  indicate  the  vigor- 
ous (and  so  successful)  branches  produced  each  season.  For 
further  discussion  consult  the  text 


spreading  crown 
of  the  adult  tree 

(Fig.  !!,/> 

So  long  as  the  central  axis  continues  to  grow  in  length,  its 
method  of  growth,  as  well  as  that  of  the  vigorous  lateral  branches, 
follows  the  general  plan  already  outlined  for  the  pine.  The  ter- 
minal twigs  at  the  ends  of  the  main  branches  produce  each  year 


22 


GENERAL  BOTANY 


clusters  of  vigorous  buds,  as  in  the  pine.  In  the  elm  the  sub- 
terminal  bud  of  each  cluster  early  replaces  the  true  terminal  bud 
and  continues  the  growth  of  the  main  axis  of  the  branch  or,  in 
the  young  tree,  of  the  central  trunk.  Two  or  three  vigorous  buds 

below  the  subterminal 
one  form  vigorous  lateral 
branches  corresponding 
to  the  false  whorls  of 
the  pine.  These  vigorous 
lateral  branches  consti- 
tuting a  false  whorl  in 
the  elm  are  not,  how- 
ever, so  numerous  as  in 
the  pine  and  are  sepa- 
rated by  longer  inter- 
nodes  (Figs.  11  and  12). 
In  the  elm  and  other  sim- 
ilar trees  some  of  the  buds 
on  the  terminal  twigs 
of  the  season  below  the 
terminal  cluster  of  vigor- 
ous buds  usually  form 
small  dwarfed  twigs, 
while  others  remain  la- 
tent. These  smaller  lat- 
eral twigs  are  ultimately 
self -pruned,  owing  to  the 
fact  that  they  are  shaded 
by  their  more  vigorous 
competitors,  so  that  the  mature  portions  of  the  trunk  and  branches 
ultimately  present  a  condition  similar  to  that  which  exists  in  the 
pines,  with  alternating  false  whorls  of  lateral  branches  and  naked 
segments  of  the  central  axis.  The  false  whorls  in  the  elm  are 
represented,  however,  by  not  more  than  two  or  three  successful 
branches  in  a  whorl,  instead  of  by  many  as  in  the  pine. 

Often,  as  the  elm  grows  older,  a  single  vigorous  branch  grows, 
each  season,  just  below  the  bud  which  continues  the  main  axis, 


FIG.  12.    A  mature  elm  tree  illustrating  the 
spreading  habit 

The  numbers  on  the  main  trunk  and  branches  are 
similar  to  those  in  Fig.  11 


THE  PLANT  BODY  23 

so  that  a  forking  appearance  is  produced  in  the  upper  branches 
and  the  terminal  twigs  of  the  crown. 

In  other  hardwood  trees,  like  the  hard  maple  and  Carolina 
poplar,  false  whorls  of  branches  "are  formed,  resembling  very 
closely  those  of  the  pine  in  distinctness  and  in  the  number  of 
branches  in  a  whorl. 

The  leaves  of  the  season  in  the  elm  and  other  hardwood  trees 
are  disposed  as  in  the  pine,  at  the  ends  of  the  terminal  twigs 
of  the  season.  In  the  spreading  type  of  tree  they  present  an  im- 
mense surface  to  the  sun  for  photosynthesis,  while  their  spiral 
or  cyclic  arrangement  secures  to  them  adequate  light  without 
the  danger  of  overlapping. 

SUMMARY 

From  the  above  accounts,  illustrated  by  the  development  of  the 
pine  and  the  elm,  it  is  seen  that  three  main  factors  determine  the  ulti- 
mate forms  of  trees  and  the  successful  display  of  their  foliage,  fruits, 
and  seeds.  These  factors  are  the  body  plan,  the  unequal  growth  of 
buds  and  branches,  and  pruning  effects  due  to  competition  in  the 
crown  of  the  tree.  Of  these  factors  the  unequal  growth  of  the  buds 
is  certainly  the  most  important,  since  by  this  factor  the  pyramidal  or 
the  spreading  form  of  the  tree  is  determined,  as  well  as  the  nature 
and  disposition  of  the  false  whorls  of  vigorous  lateral  branches. 
In  the  spreading-tree  types  the  use  of  the  term  false  whorls  is  only 
permissible  in  order  to  make  clear  the  close  similarity  which  exists 
between  the  mode  of  annual  bud  growth  in  the  pyramidal  pines  and 
that  in  the  spreading  types  of  deciduous  trees. 

In  herbaceous  plants  the  same  general  principles  obtain  in  the 
development  of  the  mature  plant  as  in  trees,  and  the  forms  assumed 
by  them  correspond,  as  we  have  seen,  to  the  erect  pyramidal  type 
and  the  spreading  type  of  the  pine  and  the  elm. 

ADJUSTMENTS  TO  THE  ENVIRONMENT  BY  TROPISMS 

It  has  just  been  shown  that  the  inherited  plan  of  the  plant 
body  of  our  common  plants  is  favorable  to  the  proper  plac- 
ing of  leaves,  roots,  and  steins  for  the  absorption  of  food 
materials  and  energy  from  the  soil,  air,  and  sunlight.  It  will 
be  quite  evident,  however,  to  the  critical  observer  of  plants, 


24 


GENEEAL  BOTANY 


Cotyledon 


that  the  growth  of  branches  and  leaves  at  regular  nodal  inter- 
vals, and  their  cyclic  and  spiral  arrangements  on  the  main  stem 
and  its  branches,  are  not  all  that  is  necessary  for  the  proper 
adjustment  of  these  organs  to  the  environment.  This  fact  is 
most  strikingly  illustrated  in  the  growth  of  young  plants  from 
the  seed,  where  the  parts  of  the  embryo,  originally  folded  in 

the  seed,  must 
gradually  expand 
and  adjust  them- 
selves to  the  air, 
light,  and  soil  for 
the  absorption  and 
use  of  raw  food  ma- 
terials. The  growth 
and  development 
of  the  embryo  and 
seedling  of  the 
common  pea  fur- 
nishes a  good  il- 
lustration of  the 
movements  which 
take  place  in  the 
adjustment  of  a 
young  plant  to 
the  new  condi- 
tions presented  to  it  as  it  emerges  from  the  seed  and  the  soil. 
The  seed  of  the  pea  is  composed  of  a  dense  outer  covering,  or 
seed  coat,  inclosing  the  embryo  proper,  which  entirely  fills  the 
seed  coats.  This  embryo  consists  of  two  fleshy  cotyledons,  or 
food-storing  leaves,  which  comprise  the  bulk  of  the  embryo. 
Between  the  two  cotyledons  are  found  two  or  three  minute 
leaves,  constituting  the  first  bud  of  the  young  plantlet,  and  a 
stemlike  body,  the  hypocotyl,  which  bears  the  cotyledons  and 
the  plumule.  These  parts  are  all  shown  in  their  proper  relation 
in  the  early  stages  of  germination  of  the  pea  (Fig.  13,  a),  just 
after  the  plumule  and  hypocotyl  have  broken  through  the 
seed  coat. 


FIG.  13.    Stages  in  the  development  of  a  seedling  of  the 
garden  pea 

a-c,  emergence  of  the  embryo  from  the  seed  and  of  the 

plumule  from  the  soil;   c-e,  erection  of  the  plumule  and 

growth  of  the  lateral  roots ;  /,  mature  seedling  with  leaves 

and  roots  adjusted  to  light  and  soil 


THE  PLANT  BODY  25 

As  the  embryo  emerges  from  the  seed  the  plumule  is  curved 
and  the  leaves  are  seen  to  be  borne  on  the  first  internode  of  the 
stem  above  the  cotyledons,  which  is  known  as  the  epicotyl.  The 
hypocotyl  has  meanwhile  elongated  and  is  continued  as  the  pri- 
mary root.  The  curved  condition  of  the  plumule  protects  its  deli- 
cate leaves  from  being  broken  off  as  it  pushes  up  through 
the  soil.  The  curvature  is  not  due  to  gravity  but  to  unknown 
internal  causes. 

As  soon  as  the  embryo  emerges  from  the  soil,  the  plumule, 
now  composed  of  several  nodes  and  internodes,  straightens  and 
finally  assumes  an  erect  position.  As  the  leaves  expand  they 
respond  to  the  stimulus  of  light  and  take  up  a  favorable  posi- 
tion for  the  reception  of  the  maximum  amount  of  light  for  the 
manufacture  of  starch. 

The  roots,  meanwhile,  respond  to  gravity  in  such  a  manner 
that  the  lateral  roots  grow  out  at  an  angle  from  the  vertical 
taproot  and  so  permeate  a  large  area  of  soil  from  which  to  draw 
water  and  soil  salts. 

The  mature  seedling  (Fig.  13,/),  through  these  various  ad- 
justing movements,  is  thus  admirably  adapted  to  securing  from 
the  environment  the  energy,  gases,  water,  and  salts  necessary 
to  its  growth  and  development. 

It  is  quite  evident  that  the  above  movements  and  changes 
in  the  position  of  the  various  organs  of  the  growing  seedling 
are  quite  independent  of  the  division  of  the  stem  into  nodes 
and  internodes  and  of  the  cyclic  or  spiral  arrangement  of  the 
leaves.  These  structural  arrangements  are,  however,  closely  cor- 
related to  the  adjusting  movements,  so  that  the  architectural 
plan  and  the  unfolding  movements  work  together  for  a  better 
final  adjustment  of  the  organs  in  the  older  seedling. 

Mature  plants  do  not  usually  manifest  such  definite  and  obvi- 
ous movements  as  seedlings,  and  yet  there  is  abundant  evidence 
in  adult  plants  of  all  kinds  that  the  ultimate  position  of  their 
growing  organs,  and  their  more  perfect  adjustments  to  the 
environment,  are  brought  about  by  a  correlation  of  body  plan 
with  adjusting  movements  which  are  caused  by  external  and 
internal  stimuli. 


26  GENERAL  BOTANY 

One  of  the  principal  reasons  why  such  adjusting  movements 
are  necessary  is  the  fact  that  the  foods  and  energy  which  must 
be  absorbed  by  green  plants  for  their  maintenance  are  usually 
quite  unequally  distributed  in  the  air  and  soil  about  an  indi- 
vidual plant.  The  plant  must  therefore  turn  its  parts  toward 
the  most  favorable  source  of  supply  to  reap  the  maximum  benefit 
of  any  given  condition.  Thus,  the  two  sides  of  a  plant  before  a 
window  are  exposed  to  very  unequal  intensities  of  light,  and  in 


6 
FIG.  14.  Phototropic  response  in  leaves  of  Nasturtium  tropaeolum 

a,  a  plant  grown  under  normal  illumination  in  a  greenhouse ;  6,  the  same  plant  after 
exposure  for  six  hours  to  lateral  illumination.   From  Cowles's  "  Ecology  " 

order  to  receive  enough  of  the  sun's  energy  for  effective  work 
in  making  starchy  foods  it  must  turn  or  swing  its  leaves  in  such 
a  manner  that  their  broad  green  surfaces  shall  be  exposed  di- 
rectly to  light  coming  in  from  the  outside.  We  are  all  familiar 
with  the  fact  that  plants  under  such  circumstances  are  able  to 
turn  the  entire  leaf  surface  from  the  normal  horizontal  position 
to  an  oblique  position  (Fig.  14),  so  as  to  face  the  sources  of 
maximum  light  supply.  Under  these  circumstances  the  separa- 
tion of  the  leaves  by  internodes,  and  their  cyclic  or  spiral 
arrangement,  is  still  effective  in  preventing  shading,  but  the 
nicer  adjustment  of  the  leaves  to  unequal  light  exposure  on  the 
two  sides  of  the  organ  is  made  through  the  power  of  the  leaves 
to  respond  to  light  acting  as  a  stimulus  and  to  turn  their 


THE  PLANT  BODY  27 

faces  toward  the  most  abundant  light  supply.  In  a  similar 
manner,  roots  which  are  growing  in  soil  where  the  amount  of 
water  or  food  varies  on  different  sides  of  the  root  system  are 
able  to  turn  toward  the  maximum  food  or  water  supply,  as 
when  roots  grow  into  old  wells  and  water  pipes,  attracted  by 
the  excess  of  moisture.  These  turnings  in  response  to  stimuli  are 
called  tropisms. 

It  is  not  surprising,  therefore,  that  plants,  probably  on  account 
of  their  stationary  habit,  have  developed  a  wonderful  sensitive- 
ness to  the  forces  and  agents  of  their  environment,  which  enables 
them  to  adapt  themselves  and  their  fixed  architectural  plan  to 
the  variations  in  their  surroundings  which  might  otherwise 
harm  them. 

Investigation  has  shown  that  plants  are  sensitive  to  gravity, 
sunlight,  moisture,  soluble  substances  in  the  soil,  and  various 
other  stimulating  forces  and  materials.  Indeed,  this  ability  to 
adjust  their  organs  is  more  marked  in  plants  than  in  most  animals. 
While  plants  are  sensitive  to  a  great  variety  of  stimuli  in  their 
surroundings,  certain  forces  and  agents  are  more  prominent  as 
directing  stimuli  than  others,  and  these  will  therefore  receive 
the  most  attention  in  the  following  discussion. 

Stimulus  and  response.  Plant  stimuli,  as  indicated  above,  are 
usually  the  external  forces  and  materials  of  the-  environment. 
Any  difference  in  the  intensity  of  such  external  stimuli  or  in 
the  direction  of  their  application  to  a  plant  organ  is  capable  of 
bringing  forth  a  response  in  the  form  of  growth,  food  building, 
adjusting  movements,  or  even  the  death  of  the  organism.  We 
are  here  concerned  only  with  that  form  of  response  to  stimuli 
that  results  in  definite  movements  which  adapt  the  plant  more 
effectively  to  the  daily  and  seasonal  changes  in  its  surroundings. 
A  moment's  consideration  will  enable  the  student  to  realize  the 
great  difference  between  the  higher  plants  and  the  higher  animals 
as  regards  both  the  reception  of  an  external  stimulus  and  the 
method,  or  mechanism,  by  which  the  two  classes  of  organisms 
respond  to  stimuli.  The  higher  animals  are  furnished  with 
special  sense  organs,  such  as  the  eye  for  the  reception  of  light 
and  the  ear  for  sound ;  in  plants  the  entire  surface  of  a  leaf  or 


28 


GENEBAL  BOTANY 


a  stem  usually  receives  the  stimulus  of  light  or  of  a  mechanical 
agent.  The  nearest  approximation  to  sense  organs  in  plants  are 
certain  groups  of  cells  in  root  tips,  and  in  the  stem  tips  of  some 
particular  plants,  which  are  endowed  with  the  property  of  re- 
ceiving gravity  or  light  stimuli.  In  general,  however,  plants 
have  no  special  sense  organs  with  nerve  endings,  like  animals, 

with   which    to    receive    external 
stimuli. 

The  mechanism,  or  method  of 
response  to  stimuli,  is  likewise 
very  different  in  the  higher  plants 
and  the  higher  animals,  since 
plants  have  no  muscles,  attached 
to  a  jointed  skeleton,  for  effect- 
ing movements.  The 
most  common  method 
of  response  in  plants 
is  that  of  unequal 
growth  on  the  two 
sides,  or  on  regularly 
alternating  sides,  of 
an  organ  which  has 
been  excited  by  an 
external  stimulus.  Thus,  in  the  pea  seedling  (Fig.  13)  the 
erection  of  the  epicotyl  from  its  curved  position  was  effected 
by  the  greater  elongation  of  the  tissues  on  the  concave  side 
of  the  epicotyl  than  on  the  convex  side.  The  horizontal  spread 
of  the  leaves  is  likewise  brought  about  by  the  more  rapid 
growth  of  the  tissues  on  one  side  of  the  young  leaves  than 
on  the  other.  The  curvatures  of  roots  and  stems,  with  which 
we  shall  have  to  deal  later,  are  also  brought  about  in  the  same 
manner.  This  method  of  securing  movements  in  plants  by 
growth  is  necessarily  slower  than  the  corresponding  move- 
ments of  animals,  which  are  results  of  nerve  stimulation  and  mus- 
cular contraction,  but  it  is  nevertheless  well  suited  to  the  nature 
of  stationary  organisms  in  which  a  new  set  of  leaves  and  new 
growths  of  stems  and  roots  are  produced  each  year. 


FIG.  15.   A  compound  leaf  of  the  bean 
with  pulvini 

The  pulvini  appear  at  the  base  of  each  leaf- 
let.  Note  also  a  large  pulvinus  at  the  base 
of  the  main  petiole.   After  Sachs 


THE  PLANT  BODY 


29 


Petiol 


Petiole 


In  the  case  of  roots  the  older  portions  of  the  root  systems  are 
already  fixed  in  their  position  by  the  surrounding  solid  earth; 
but  the  new  roots  which  grow  from  the  old  ones  each  spring 
are  enabled  to  move  and  penetrate  into  new  soil  regions  in 
order  to  absorb  foods  and  water.  In  a  similar  manner  each  new 
annual  crop  of  leaves  can  adjust  itself  to  the  conditions  con- 
fronting it  in  the  season  in  which  it  must  do  its  work.  Flow- 
ers are  likewise  able  to  adjust  their  positions  so  that  they 
may  secure  the  visits  of  pollinating  insects,  and  fruits  assume 
positions  suitable  for  dissem- 
ination by  wind  or  animals. 
The  older  parts,  which  have 
lost  the  power  of  growth,  and 
therefore  of  movement,  thus 
become  supporting,  storing, 
and  conducting  organs  of 
plants,  while  the  annual 
growth  of  new  roots,  leaves, 
and  flowers  enables  the  or- 
ganisms to  adjust  them- 
selves to  any  ordinary  change 
which  may  take  place  in 
the  environment,  and  thus  to  adapt  themselves"  to  the  usual 
seasonal  changes  and  other  requirements  of  their  surroundings. 

Special  motor  organs  exist  in  some  plants,  however,  which 
enable  certain  organs  to  execute  more  rapid  and  exact  move- 
ments than  those  described  above.  Examples  of  such  motor 
organs  are  found  in  the  leaves  of  peas  and  their  near  relatives. 
These  special  organs  are  termed  pulvini  (singular,  pulvinus) 
(Figs.  15  and  16).  They  are  highly  modified  portions  of  the 
petioles  at  the  base  of  each  leaflet  arid  often  at  the  base  of 
the  petiole  itself,  where  it  joins  the  main  stem  or  a  branch. 
The  quick  curvature  of  these  pulvini  serves  to  swing  the  leaves 
into  positions  which  enable  them  to  secure  a  better  adaptation 
to  light  or  a  more  complete  protection  from  too  great  loss  of 
water.  Internal  and  unknown  stimuli  also  affect  plant  move- 
ments, in  some  instances  quite  as  profoundly  as  the  external 


FIG.  16.   Pulvini  of  the  bean 
Magnified.   After  Sachs 


30  GENERAL  BOTANY 

stimuli  discussed  above.  Thus,  the  arching  of  the  epicotyl  of 
the  pea  seedling,  already  referred  to,  is  not  due  to  external 
stimuli,  but  to  internal  conditions  which  bring  about  unequal 
growth  of  the  organ  on  two  opposite  sides,  resulting  in  an 
adaptive  curvature.  Some  of  the  adaptive  movements  of 
flowers  and  fruits  are  apparently  due  to  similar  internal  con- 
ditions which  appear  to  affect  the  organism  like  external  stimuli. 
These  latter  are,  however,  special  cases  applying  to  particular 
plant  groups  or  species  and  do  not  affect  the  general  principle 
that  plant  movements  are  usually  effected  by  unequal  growth 
in  response  to  external  stimuli. 

In  designating  the  kinds  of  stimuli  and  the  nature  of  the 
response  of  plant  organs  to  them  botanists  have  adopted  certain 
terms  which  are  useful  in  designating  stimulus  and  response  as 
applied  to  any  given  organ  of  a  plant.  Thus,  organs  may  be 
said  to  be  protropic  when  they  grow  toward  the  source  of  the 
stimulating  force,  and  apotropic  when  they  grow  away  from 
such  a  source.  Protropic  and  apotropic  organs  will  evidently 
have  their  main  axes  parallel  with  the  direction  from  which 
a  given  stimulus  acts.  Diatropic  organs,  on  the  contrary,  do 
not  grow  toward  the  source  of  a  directive  force,  but  place  their 
main  axes  at  some  angle  to  the  direction  of  a  stimulating  agent. 
If  we  apply  these  definitions  to  the  pea  seedling  (Fig.  13),  the 
main  root  would  be  protropic  and  the  stem  apotropic  with 
reference  to  gravity.  The  secondary  roots  would,  however,  be 
diatropic  with  reference  to  gravity,  and  the  leaves  diatropic 
with  reference  to  light.  If  it  is  desired  to  combine  the  name 
of  the  stimulus  with  the  nature  of  the  response,  other  terms 
.may  be  combined  with  protropic,  apotropic,  and  diatropic. 
Thus,  organs  stimulated  by  gravity  are  said  to  be  geotropic; 
if  stimulated  by  light,  they  are  designated  as  phototropic. 
Progeotropic,  apogeotropic,  and  diageotropic  may  therefore  be 
used  to  indicate  the  responses  to  a  gravity  stimulus,  on  the 
one  hand,  and  prophototropic,  apophototropic,  and  diaphototropic  to 
indicate  the  responses  to  a  light  stimulus,  on  the  other.  Similar 
combined  terms  are  used  in  connection  with  other  stimuli,  but 
these  need  not  be  considered  in  our  brief  account. 


TPIE  PLANT  BODY  31 

The  gravity  sense.  It  will  be  of  special  interest  to  the  stu- 
dent at  this  point  to  learn  something  of  the  method  by  which 
some  of  the  great  botanists  of  the  past  laid  the  foundation  for 
our  present  understanding  of  the  nature  of  plant  movements 
and  of  the  relation  which  exists  between  external  stimuli  and 
the  response  of  the  plant  to  them.  For  this  purpose  the  classi- 
cal experiments  of  Thomas  Andrew  Knight,  Julius  von  Sachs, 
and  Charles  Darwin  on  the  gravity  sense  of  plants  and  its 
relation  to  plant  movements  may  well  serve  as  an  illustration. 

Gravity  is  the  most  universal  and  constant  external  force 
acting  upon  the  living  plant  world,  and  it  is  not  surprising, 
therefore,  that  of  all  the  outside  forces  this  is  found  to  be  the 
most  potent  in  directing  the  adjustments  of  plant  organs  to 
their  environment  by  movements  or  tropisms. 

The  general  nature  of  the  response  of  plants  to  gravity  is 
suggested  by  the  fact  that  the  stems  and  roots  of  all  plants 
take  up  the  same  position  with  reference  to  the  earth's  center 
at  all  points  on  the  earth's  surface.  Thus,  plants  on  opposite 
sides  of  the  earth  are  found  to  have  the  main  root  growing 
toward  the  earth's  center  and  the  main  stem  away  from  it. 
Likewise,  growing  plants  which  have  been  prostrated  by  storms, 
or  which  happen  to  grow  on  steep  hillsides,  always  tend  to 
place  their  stems  in  a  vertical  position  with  reference  to  the 
earth's  center.  This  suggests  the  general  law  that  roots  re- 
spond to  gravity  by  growing  toward  the  earth's  center,  while 
stems  tend  to  grow  in  the  opposite  direction  in  response  to  the 
same  stimulus. 

This  general  law  was  first  tested  out  in  1806  by  Thomas 
Andrew  Knight,  an  English  botanist,  who  conceived  the  idea  of 
substituting  centrifugal  force  for  gravity,  to  see  how  roots  and 
stems  would  respond  to  other  forces  than  gravity.  Knight 
attached  boxes,  in  which  young  plants  were  growing,  to  the  cir- 
cumference of  rapidly  rotating  wheels.  When  the  wheels  were 
rotated  rapidly  enough,  he  observed  that  the  roots  grew  toward 
the  circumference  of  the  wheels,  with  the  acting  centrifugal  force, 
while  the  stems  grew  toward  the  center  of  the  wheels,  or  against 
the  acting  force.  If  the  wheels  were  rotated  less  rapidly,  the 


32  GENERAL  BOTANY 

roots  and  stems  took  up  an  intermediate  position  which  was  a 
resultant  of  the  response  of  "the  plant  to  the  two  forces,  gravity 
and  centrifugal  force,  acting  separately. 

Knight  concluded,  therefore,  that  roots  and  stems  respond  to 
centrifugal  force  acting  as  a  stimulus.  His  simple  appliances 
are  now  replaced  by  more  perfect  pieces  of  apparatus,  on  which 
disks  can  be  rotated  with  extreme  rapidity  and  accuracy.  If 
kernels  of  germinating  seeds  of  corn  are  placed  on  such  a  disk 


FIG.  17.    Diagram  illustrating   the  principle   of   an   experiment   by  Thomas 
Andrew  Knight  (1806) 

a,  position  of  seeds,  roots,  and  plumule  (stem)  at  the  beginning  of  the  experiment ; 
6,  position  of  root  and  plumule  (stem)  after  rapid  rotation.    Further  discussion  in 
explanation  of  a  and  b  in  the  text 

in  the  positions  indicated  in  Fig.  17,  a,  and  the  disk  is  then 
rotated  for  twenty-four  hours,  the  elongating  root  and  stem  will 
gradually  assume  the  positions  indicated  in  b.  The  roots  will 
all  grow  toward  the  circumference  of  the  wheels,  while  the 
stems  will  all  grow  toward  its  center. 

By  employing  a  force  which  he  could  control  and  modify, 
Knight  was  thus  able  to  show  that  the  root  and  the  stem  could 
be  caused  to  curve  and  to  take  up  various  positions  as  a  result 
of  their  response  to  this  force  acting  as  a  stimulus.  Since  seeds 
germinating  in  the  soil  seemed  to  behave  toward  gravity  as  they 
did  toward  centrifugal  force  in  his  experiment,  he  concluded  that 
gravity,  acting  as  a  stimulus,  directed  the  growth  of  the  root 
toward  the  earth's  center  and  the  stem  away  from  it,  and  that 


THE  PLANT  BODY 


33 


the  opposite  positions  assumed  by  these  organs  on  the  earth's 
surface  were  due  to  gravity  acting  as  the  stimulating  agent. 

It  is  a  matter  of  common  experience  that  if  germinating  seeds 
are  placed  horizontally  in  soil  or  with  the  root  pointing  upward 
and  the  stem  downward,  the  growing  organs  turn  and  adjust 
themselves  to  grav- 
ity, as  they  do  to 
centrifugal  force  on 
rotating  disks. 

Lateral  organs  such 
as  branches  and  lat- 
eral roots  have  also 
been  found  to  be  gov- 
erned very  largely, 
in  the  position  which 
they  finally  assume, 
by  gravity,  although 
other  forces  are  often 
influential  in  the  ulti- 
mate adjustment. 

Knight's  early  con-      FlG<  lg>   Diagrams  illustrating  the  principle  of  an 
elusions  were  proved  experiment  by  Julius  von  Sachs  (1879) 

to  be  correct  by  the 
great  German  botan- 
ist Julius  von  Sachs 
in  1879.  Sachs  used 
a  different  method 
for  proving  that  grav- 


a,  the  position  of  germinating  seeds  of  corn  on  a  disk, 
which  is  then  slowly  rotated  for  several  hours  in  a 
vertical  position ;  b,  the  positions  assumed  by  roots  and 
plumule  (stem)  after  twenty-four  hours  of  rotation ; 
c,  the  position  of  germinating  seeds  of  corn  on  a  station- 
ary disk  at  the  beginning  of  the  experiment;  d,  the 
positions  assumed  by  roots  and  plumule  (stem)  after 
twenty-four  hours  without  rotation.  Consult  the  text 
for  a  discussion  of  this  experiment 


ity  acts  upon  plants 
as  a  directive  stimulating  force.  He  placed  growing  seedlings 
on  slowly  rotating  vertical  wheels  or  disks  (Fig.  18).  As  the 
wheels  revolved,  the  stimulus  of  gravity  continued,  but  the 
effect  of  gravity  on  the  growing  stem  and  root  was  practically 
eliminated,  since  it  acted  for  too  short  an  interval  of  time  on 
any  given  side  of  these  organs  to  secure  a  reaction  in  the  form  of 
a  curvature.  Plant  organs  usually  have  to  remain  in  a  position 
of  stimulation  from  thirty  minutes  to  several  hours  in  order  to 


34  GENERAL  BOTANY 

result  in  curvature.  If,  therefore,  seedlings  are  rotated  so  that 
opposite  sides  of  the  stem  and  root  are  alternately  placed  in  a 
position  of  stimulation  for  shorter  periods  than  are  required  for  a 
reaction,  the  organs  will  fail  to  respond  to  gravity.  The  time  dur- 
ing which  an  organ  like  a  root  must  be  continuously  stimulated 
on  one  side  in  order  to  secure  a  reaction  is  called  presentation 
time.  In  Sachs's  experiment  the  presentation  time  to  gravity  was 
too  short  in  any  given  position  of  the  growing  stem  and  root  tip 

to  secure  curvature,  and  these 
c~~^ — }  a      organs  therefore  grew  in  any 
(/*  — i  direction  in  which  they  were 

placed  at  the  beginning  of 
the  rotation.  This  proved 
definitely  the  effect  of  grav- 

Fio.19.    An  experiment  by  Charles  Dar-      ^    aS    a    directiv^    force   in 

win,    designed   to  locate   the    sensitive      the     growth     of     roots     and 

portion  of  the  root  to  gravity  stems  in  the  normal  vertical 

a,  uninjured  roots  of  the  bean  extended  position.    It  will  thus  be  Seen 
horizontally   for   twenty-three    hours   and  ,          T^    .    ,              ,    0      , 
thirty  minutes;    b,  root  tips  of  the  bean  that    Knight    and    Sachs    em- 
touched   with  caustic  and   extended  hori-  ployed  quite  different  metll- 
zontally  for  the  same  length  of  time  as  ,        .          ,     . 

those  in  a.    After  Charles  Darwin  ods     111     their      experiments. 

Knight   aimed  to   substitute 

another  force  for  gravity  in  order  to  note  its  effect  on  root  and 
stem  growth,  while  Sachs  sought  to  neutralize  and  so  eliminate 
the  effects  of  gravity.  Knight's  experiment  proved  that  roots 
and  stems  respond  to  an  external  force  which  can  be  controlled 
and  its  effects  therefore  proved,  while  Sachs's  experiment  showed 
conclusively  that  gravity  was  necessary  for  the  downward 
growth  of  roots  and  the  upward  growth  of  the  stem. 

Darwin's  work  entitled  "  Tho  Power  of  Movement  in  Plants  " 
greatly  extended  the  observations  already  made  on  the  sensitive- 
ness of  the  root  to  gravity  and  other  stimuli.  His  most  important 
contribution  to  the  subject  was  the  elaboration  of  the  idea, 
already  discovered  by  Sachs,  that  the  sensitive  zone  is  located 
in  the  very  apex  of  the  root.  Fig.  19  illustrates  Darwin's  method 
of  locating  the  sensitive,  or  perceptive,  zone  of  the  root  in  the 
root  tip  of  the  common  garden  bean  ( Vicia  faba).  Darwin's 


THE  PLANT  BODY  35 

conclusion,  given  in  a  summary  of  his  chapter  on  the  "  sensi- 
tiveness of  the  radicle  (root)  to  contact  and  other  irritants," 
indicates  the  nature  of  his  contribution  to  the  general  subject 
of  the  response  of  the  root  to  various  stimuli,  including  gravity, 
in  its  natural  growth  through  the  soil. 

The  peculiar  form  of  sensitiveness  which  we  are  here  considering 
is  confined  to  the  tip  of  the  radicle  for  a  length  of  from  1  mm.  to 
1.5  mm.  When  this  part  is  irritated  by  contact  with  any  object,  by 
caustic,  or  by  a  thin  slice  being  cut  off,  the  upper  adjoining  part  of 
the  radicle,  for  a  length  of  from  6  or  7  to  even  12  mm.,  is  excited 
to  bend  away  from  the  side  which  has  been  irritated.  Some  influ- 
ence must  therefore  be  transmitted  from  the  tip  along  the  radicle 
for  this  length.  The  curvature  thus  caused  is  generally  symmetrical. 
The  part  which  bends  most  apparently  coincides  with  that  of  the 
most  rapid  growth.  The  tip  and  the  basal  part  grow  very  slowly, 
and  they  bend  very  little. 

Considering  the  several  facts  given  in  this  chapter,  we  see  that  the 
course  followed  by  a  root  through  the  soil  is  governed  by  extraordi- 
narily complex  and  diversified  agencies, —  by  geotropism  acting  in 
a  different  manner  on  the  primary,  secondary,  or  tertiary  radicles ; 
by  sensitiveness  to  contact,  different  in  kind  in  the  apex  and  in 
the  part  immediately  above  the  apex ;  and  apparently  by  sensitive- 
ness to  the  varying  dampness  of  different  parts  of  the  soil.  These 
several  stimuli  to  movement  are  all  more  powerful  than  geotropism, 
when  this  acts  obliquely  on  a  radicle  which  has  been  deflected  from 
its  perpendicular  downward  course.  The  roots,  moreover,  of  most 
plants  are  excited  by  light  to  bend  either  to  or  from  it ;  but  as  roots 
are  not  naturally  exposed  to  the  light,  it  is  doubtful  whether  this 
sensitiveness,  which  is  perhaps  only  the  indirect  result  of  the  radi- 
cles' being  highly  sensitive  to  other  stimuli,  is  of  any  service  to  the 
plant.  The  direction  which  the  apex  takes  at  each  successive  period 
of  the  growth  of  a  root  ultimately  determines  its  whole  course ;  it 
is  therefore  highly  important  that  the  apex  should  pursue  from  the 
first  the  most  advantageous  direction  ;  and  we  can  thus  understand 
why  sensitiveness  to  geotropism,  to  contact,  and  to  moisture  all  re- 
side in  the  tip,  and  why  the  tip  determines  the  upper  growing  part 
to  bend  either  from  or  to  the  exciting  cause.  A  radicle  may  be  com- 
pared with  a  burrowing  animal  such  as  a  mole,  which  wishes  to 
penetrate  perpendicularly  down  into  the  ground.  By  continually 


36 


GENERAL  BOTANY 


moving  his  head  from  side  to  side,  or  circumnutating,  he  will  feel 
any  stone  or  other  obstacle,  as  well  as  any  difference  in  the  hardness 
of  the  soil,  and  he  will  turn  from  that  side ;  if  the  earth  is  damper 
on  one  than  on  the  other  side,  he  will  turn  thitherward  as  a  better 
hunting-ground.  Nevertheless,  after  each  interruption,  guided  by 
the  sense  of  gravity,  he  will  be  able  to  recover  his  downward  course 
and  to  burrow  to  a  greater  depth. 

Darwin's  contributions  to  our  knowledge  of  the  sensitiveness 
of  the  root  tip  have  been  confirmed  by  later  researches  and  have 

done  much  towards 
elucidating  our  ideas 
concerning  the  nature 
of  stimulus  and  re- 
sponse as  applied  to 
plants.  Fig.  20  illus- 
trates the  principle  of 
some  ingenious  experi- 
ments by  Czapek,  who 
confirmed  Darwin's  gen- 
eral idea  that  the  sen- 
sitiveness of  the  root 
to  gravity  is  located 
mainly  in  the  root  tip. 
Czapek  forced  roots  to 
grow  into  bent  tubes, 
as  illustrated  in  the 
figure,  so  that  the  last 
millimeter  of  the  root  was  at  right  angles  to  the  body  of  the  root. 
Such  roots,  when  placed  in  the  position  indicated  in  Fig.  20,  a, 
produced  a  curvature  like  that  in  b.  Root  tips  placed  in  a 
position  like  that  of  c,  however,  failed  to  curve  in  response 
to  gravity,  as  indicated  in  <#,  since  the  sensitive  tip  was  in  the 
normal  progeotropic  position.  When,  therefore,  the  terminal 
millimeter  of  the  root  tip  is  placed  in  a  position  of  stimulation 
(Fig.  20,  a),  curvature  of  the  root  occurs ;  but  when  the  terminal 
millimeter  is  placed  vertically  (<?),  no  curvature  results.  We  may 
conclude,  therefore,  with  Czapek,  that  the  last  millimeter  of  the 


FIG.  20.    Drawings  illustrating  experiments  by 

Czapek,  designed  to  locate  the  sensitive  zone  of 

the  root  to  gravity  acting  as  a  stimulus 

The  roots  of  the  bean  were  grown  into  glass  slippers 
and  then  placed  so  as  to  expose  the  last  few  milli- 
meters of  the  tip  to  gravity  at  different  angles  from 
the  vertical.  «,  position  of  the  root  tips  at  the  begin- 
ning of  the  expei-iment;  6,  position  assumed  by  the 
above  root  tips  twenty  hours  later  -  c,  root  tips  placed 
vertically  at  the  beginning  of  the  experiment :  d,  posi- 
tion assumed  by  the  same  roots  eighteen  hours  later. 
Further  discussion  in  the  text.  How  do  you  explain 
the  difference  in  the  position  assumed  by  roots  in  b 
and  d  ?  After  Czapek 


THE  PLANT  BODY  37 

root  tip  is  the  most  sensitive  part  of  the  root  and  is  the  perceptive 
region  for  the  gravity  stimulus  which  causes  curvature. 

We  may  now  profitably  turn  to  a  few  common  garden  and  field 
plants  for  a  practical  application  of  the  principles  established  above 
with  reference  to  the  adjustment  of  the  plant  parts  in  response  to 
stimuli  and  consider  briefly  the  part  which  these  responses  play 
in  determining  the  attitudes  and  ultimate  form  of  these  plants. 

ADJUSTMENTS  IN  SOME  COMMON  PLANTS 

Caladium  and  clover.  The  power  of  two  quite  different  plants 
to  adjust  their  organs  to  the  environment  may  be  seen  by  con- 
trasting the  ultimate  positions  assumed  by  the  organs  of  cala- 
dium  and  red  clover  (Figs.  21  and  22).  These  two  plants  have 
very  different  forms  of  roots  and  leaves,  and  yet  each  plant,  on 
account  of  its  ability  to  respond  in  its  own  way  to  the  forces 
of  its  environment,  has  succeeded  in  placing  its  roots  and  leaves 
in  the  position  most  favorable  to  its  own  maintenance  and 
growth.  As  may  be  seen  from  Fig.  21,  the  caladium  is  what 
is  known  as  a  surface  feeder,  spreading  its  roots  out  horizontally 
and  absorbing  foods  and  moisture  from  the  surface  layers  of  the 
soil.  Many  desert  plants  arrange  their  roots  in  this  manner  in 
order  to  avail  themselves  of  the  light  rains  and  heavy  dews 
which  occur  in  arid  regions  at  certain  times  of  the  year.  The 
leaves  of  the  oaladium  are  also  favorably  adjusted  to  light  as  a 
result  of  their  ability  to  turn  in  response  to  the  light  stimulus. 
The  roots  are  evidently  diageotropic  and  the  leaves  diaphoto- 
tropic  in  their  response. 

The  common  red  clover  (Trifolium pratense)  (Fig.  22)  presents 
a  strong  contrast  to  the  caladium  not  only  in  the  form  of  its 
organs  but  also  in  its  responses  to  light  and  gravity.  The  strong 
taproot  is  here  progeotropic  and  bores  deeply  into  the  soil,  so 
that  the  lateral  diageotropic  roots  absorb  food  and  moisture  from 
much  deeper  areas  than  the  similar  roots  of  the  caladium.  The 
leaves  of  the  clover,  like  those  of  the  bean,  are  furnished  with 
pulvini  (5)  and  thus  adjust  themselves  rapidly  and  effectively 
to  changes  in  the  external  environment.  During  the  day  they 


38 


GENERAL  BOTANY 


are  expanded  to  the  sun,  while  at  night  they  fold  up  (Fig.  28), 
assuming  the  so-called  nyctitropic,  or  sleep,  position. 

The  mechanism  of  this  curvature  is  supposed  to  consist  in  an 
unequal  absorption  of  water  on  two  opposed  sides  of  the  pul- 
vinus  as  a  response  to  different  intensities  of  light,  as  in  the 


FIG.  21.    Positions  assumed  by  the  leaves  and  roots  of  a  mature  caladium  plant 
in  response  to  light  and  gravity.    After  Koerner 

leaves  of  bean  plants,  which  may  be  seen  to  fall  and  assume  a 
vertical,  hanging  position  at  night,  owing  to  the  greater  increase 
in  length  of  the  upper  side  of  the  pulvinus  as  compared  with  the 
lower  side,  thus  inducing  curvature  and  the  downward  bending 
of  the  leaves  (Fig.  16).  This  method  of  curvature  is  not  unlike 
that  of  roots  and  petioles  already  described,  except  that  in  these 


THE  PLANT  BODY 


39 


cases  curvature  is  due  to  slow,  unequal  growth  on  opposite  sides 
of  a  part,  while  in  pulvinar  movements  the  curvature  is  due  to 
rapid  temporary  growth  caused  by  the  unequal  inflation  with 


FIG.  22.    Positions  assumed  by  the  leaves  and  roots 
of  a  young  plant  of  red  clover  in  normal  light 

a,  the  entire  plant;  b,  three  leaflets  with  pnlvini 


Pulvinus 
Petiole 


water  of  the  cells  on 
the  upper  and  lower 
sides  of  the  pul- 
vinus.  This  highly 
specialized  type  of 
leaf  movement  by 
pulvini,  observed 
above  in  the  bean 
and  the  red  clover, 
is  characteristic  of 
the  entire  pea  family 
(Leguminosae),  to 
which  they  belong. 
Alfalfa,  the  com- 
mon clovers,  locust, 
peas,  beans,  and  the 
sensitive  plant  (Mi- 
mosa) are  other  well- 
known  members  of 
this  very  important 
family  of  plants.  The 
contrast  between  the 
caladium  and  the 


clover  plants  outlined  above  emphasizes  the  fact  that  each  plant  in 
any  given  environment  is  able  to  adjust  its  organs  by  movements 
so  as  to  adapt  them  to  surrounding  conditions.  The  amount  and 


40  GENERAL  BOTANY 

manner  of  adjustment  varies  in  different  plants,  but  all  plants 
secure  a  fair  adaptation  to  the  environment  in  which  they  live. 
The  dandelion.  In  the  dandelion  (Taraxacum  officinale) 
(Fig.  24)  the  root  system  resembles  that  of  the  clover  plant  in 
the  form  of  the  main  taproot  and  in  the  pro  tropic  and  diatropic 
responses  of  the  main  and  lateral  roots  to  gravity.  The  leaves 
are  diaphototropic  and  are  arranged  in  the  form  of  a  rosette, 
with  the  smaller  leaves  in  the  center  alternating  with  the  larger, 
outside  leaves. 

The  scapes,  or  supporting  stalks,  of  the  flower  clusters  of 
the  dandelion,  however,  manifest  a  great  variety  of  responses  to 

gravity,  and  these  movements 
are    evidently    closely   corre- 
lated with  the  various  stages 
in    the    development    of   the 
flowers  and  the  fruit.    They 
thus    furnish    an    interesting 
example  of  the  fact,  often  ob- 
FIG.  23.    Corresponding    positions    as-     served   in    plant   movements, 
sumed  by  the  leaflets  of  red  clover  during     t^at  t^e  nature  of  the  response 
the  day  and  at  night  ,, 

or  a  given  organ  to  an  out- 

a.  position  of  the  leaflets  in  normal  daylight;        -j      •    a  .•        i 

6,  positions  of  the  same  leaflets  at  night.      Slde  influence,   or  stimulus,  IS 

Note  that  the  position  in  6  is  assumed  by     apparently  determined,  at  any 

curvature  of  the  pulvini  ,        . ,  f 

given  time,  by  the  stage  of 

development  and  the  internal  condition  of  the  organ  or  part.  The 
dandelion  scapes  are  sensitive  to  light,  but  the  movements  here 
figured  and  discussed  are,  for  the  most  part,  controlled  by  gravity, 
and  hence  the  light  stimulation  is  not  taken,  in  to  account. 

In  Fig.  24  the  various  stages  in  the  development  of  a  single 
flower  cluster  and  its  fruit  are  shown,  beginning  with  the  bud 
stage  (a)  and  ending  with  the  final  seed-shedding  stage  (A). 
By  placing  the  plant  upon  a  slowly  rotating  disk  connected 
with  a  clinostat  it  can  be  determined  that  the  various  positions 
assumed  by  the  scapes  in  the  figure  are  a  result  of  the  response 
of  these  scapes  to  gravity.  The  scapes  are  therefore  either 
apogeotropic  or  diageotropic  at  different  stages  of  development. 
In  the  early  bud  stage  (a)  the  scape  is  short  and  the  bud  is 


THE  PLANT  BODY 


41 


apogeotropic  and  erect.  As  the  bud  grows  the  scape  elongates 
and  becomes  temporarily  diageotropic,  as  indicated  in  b  and  c. 
When  the  flowers  open,  the  scape  and  flower  cluster  become  apo- 
geotropic, and  the  erect  flowers  are  thus  exposed  favorably  for 
cross-pollination  by  insects.  Curiously  enough,  in  the  dandelion 
the  necessity  for  cross-pollination  seems  to  have  been  lost  in  its 


Seed  shedding 


Flowers 


FIG.  24.  The  response  of  the  organs  of  the  dandelion  to  light  and  gravity 

Note  and  be  able  to  explain  the  positions  assumed  by  the  leaves  and  roots.    The 

different  positions  of  the  flower  stalk  (scape)  of  the  flowering  head  are  indicated 

by  the  letters  a-h.   Consult  the  text  for  further  discussion 

later  history,  since  it  has  been  found  that  its  seeds  may  develop 
without  fertilization,  though  insect  pollination,  and  its  useful 
effects,  is  not  thereby  precluded.  When  the  seeds  begin  to  form,  the 
scape  moves  downward  (e  and/),  becoming  again  diageotropic  in 
its  response  to  gravity,  so  that  the  seeds  develop  near  the  ground 
(/),  in  a  more  protected  position  than  the  upright,  apogeotropic 
position  of  the  open  flowers  (c?).  When  the  seeds  are  ripe  and 
ready  to  be  shed,  the  scape  once  more  changes  its  responses  to 
the  gravity  stimulus  from  the  diageotropic  position  (/)  and 


42  GENERAL  BOTANY 

quickly  moves  upward  again  to  the  apogeotropic  position  (A), 
which  is  favorable  to  the  distribution  of  the  seeds.  Each  seed, 
thus  exposed  to  air  currents,  is  furnished  with  a  parachute 
apparatus  composed  of  fine  hairs  which  grow  out  from  the 
end  of  a  long,  stalklike  beak  projecting  from  the  seed.  This 
hairy  parachute  is  also  sensitive  to  moisture,  closing  up  in 
wet  weather  and  opening  out  on  dry  days,  when  the  seeds 
may  more  easily  be  disseminated.  The  envelopes  of  the  entire 
flower  cluster  are  also  extremely  sensitive  to  light,  moisture, 
and  temperature  and  assume  a  closed  or  an  open  position  dur- 
ing different  stages  in  the  development  of  the  flower  cluster,  as 
may  be  seen  in  Fig.  24.  The  flower  clusters  are  also  closed  at 
night  and  on  cold  days,  and  open  in  warm,  bright  weather. 

We  see,  therefore,  in  the  dandelion  a  remarkable  variety  of 
movements,  most  of  which  appear  to  be  direct  responses  of 
the  various  organs  and  their  parts  to  stimuli  induced  by 
gravity,  light,  moisture,  and  temperature.  By  means  of  these 
responses  the  organism  is  not  only  able  to  adjust  its  organs 
to  the  daily  fluctuations  of  the  environmental  forces  and  con- 
ditions but  can  also  adapt  its  responses  to  the  needs  of  par- 
ticular organs  at  different  stages  in  their  development.  Thus, 
the  opening  and  closing  of  the  flower  and  fruit  heads  may  be 
a  daily  response  and  -adaptation,  but  the  various  positions 
assumed  by  the  scapes  extend  over  many  days  and  even  weeks 
and  are  determined  by  some  unknown  coordination  between  the 
state  or  condition  of  the  organ  and  its  power  of  response  to 
gravity.  Many  other  instances  of  similar  adjustments  of  floral 
parts  to  environmental  conditions  might  be  mentioned,  but  they 
are  all  similar,  in  their  general  nature,  to  that  of  the  dandelion 
and  need  not  be  considered  here. 

Trees  and  shrubs.  In  trees  and  shrubs,  as  we  have  already 
learned,  the  ultimate  form  is  assumed  as  a  result  of  the  general 
plan  of  the  plant  body  combined  with  adjusting  movements. 
The  cohelike  aspect  of  a  pine  tree,  for  example,  is  an  excellent 
illustration  of  the  truth  of  the  above  statement.  In  the  pine 
(Fig.  10,  5)  the  main  stem  remains  apogeotropic  and  continues  its 
vigorous  growth  during  the  life  of  the  tree.  The  lateral  branches, 


THE  PLANT  BODY 


43 


which  arise  at  regular  intervals  determined  by  the  mathematical 
spiral  arrangement  of  the  leaves  and  buds,  are  diageotropic  and 
diaphototropic  and  thus  assume  a  horizontal  position. 

The  influence  of  light  in  modifying  the  attitude  of  the  pine 
tree  and  its  branches  is  also  well  illustrated  in  the  figure.  At 
the  apex  of  the  tree  the  young  branches  assume  a  more  or 

less  uniform  upward 

direction,  due  to  the 
combined  effect  of 
gravity  and  light. 
On  the  lower  por- 
tions of  the  trunk  the 
branches  are  diageo- 
tropic and  nearly  hor- 
izontal near  the  tree 
trunk,  but  farther 
out  toward  the  apex 
they  curve  rather 
sharply  upward,  be- 
coming nearly  pro- 
phototropic  at  the 
extreme  end  of  the 
branch.  Careful  in- 
spection of  such  a 
tree  will  demonstrate 

Note  the  mixed  response  of  the  branches  to  light  and 
the    fact    that    these       gravity.  Consult  the  text  for  a  discussion  of  this  figure 

various  attitudes  as- 
sumed by  the  branches  of  the  pine,  due  to  adjusting  movements 
in  the  response  to  light  and  gravity  stimuli,  secure  the  best 
possible  exposure  of  the  terminal  tufts  of  needle  leaves  to 
light.  In  a  similar  manner  the  ripening  seeds  in  the  pine  cones 
are  exposed  to  air  currents  for  dissemination. 

Exactly  similar  phenomena  may  be  observed  in  young  and 
adult  trees  of  the  spreading  type  (Fig.  25),  where  the  ultimate 
form  of  the  branches  and  the  placing  of  the  leaves  are  deter- 
mined by  responses  to  light  and  gravity  acting  as  stimuli, 
The  form  of  all  trees  and  the  proper  exposure  of  their  leafage 


FIG.  25.   The  form  and  position  of  the  branches  of 
the  American  elm 


44  GENERAL  BOTANY 

and  fruit  are,  therefore,  determined  by  the  three  factors  so 
frequently  referred  to  in  the  previous  pages,  namely,  body 
plan,  bud  growth,  and  adjusting  movements,  or  tropisms. 

These  few  illustrations  of  the  adjusting  movements  of  some 
common  plants  will  enable  the  student  to  interpret  similar 
phenomena  in  the  great  variety  of  plant  forms  which  enter 
into  his  daily  experience.  The  simple  account  given  above 
may  also  stimulate  interest  in  the  movements  of  plant  parts, 
and  will  furnish  a  clearer  realization  of  the  important  factors 
to  which  the  ultimate  forms  and  attitudes  of  common  plants 
are  due. 

SUMMARY 

Plants,  like  animals,  are  sensitive  to  stimuli  acting  upon  the  living 
substance  of  the  plant  organism. 

The  nervous  mechanism  of  plants  is,  however,  lodged  in  the  living 
substance  of  growing  organs,  and  there  is  no  evidence  of  the  differ- 
entiation within  the  plant  body  of  a  definite  nervous  system  corre- 
sponding to  the  nerve  fibers  and  nerve  cells  of  the  higher  animals. 
The  reception  of  stimuli  and  the  transmission  of  impulses  in  plants 
take  place,  therefore,  through  the  living  substance  of  plant  organs, 
which  is  undoubtedly  so  connected  as  to  establish  a  continuity  of 
living  substance  throughout  sensitive  and  moving  parts. 

The  mechanism  of  movement  also  differs  in  plants  and  animals, 
being  confined  in  plants  to  growing  parts  in  which  the  unequal 
growth  on  different  sides  of  an  organ  can  effect  curvature  and  move- 
ments such  as  we  have  observed  above  in  roots,  stems,  and  leaves. 

The  function,  or  use,  of  the  sensitiveness  of  plants  and  their  power 
to  react  to  stimuli  is  found  in  the  adjustment  of  their  organs  to 
the  environment.  For  this  reason  plants  have  developed  a  special 
sensitiveness  to  such  stimuli  as  light  and  gravity,  which  enables 
them  to  adjust  their  organs  to  advantageous  positions  for  the 
absorption  of  raw  materials,  the  manufacture  of  food,  and  the  produc- 
tion of  flowers,  fruits,  and  seeds.  Highly  sensitive  living  substance 
for  the  reception  of  stimuli  is  found  in  root  tips,  in  some  stem 
tips,  and  possibly  in  some  leaf  surfaces.  Specialized  motor  organs 
are  also  developed  in  some  plant  families.  Plants  as  a  whole,  how- 
ever, while  sensitive  to  a  greater  variety  of  stimuli  than  animals, 
are  far  below  the  latter  in  special  sense  organs  and  in  nervous  organi- 
zation for  receiving  and  responding  to  external  and  internal  stimuli. 


SECTION  II.    CELL  STRUCTURE  AND 
ANATOMY 

CHAPTER  III 

THE  CELLULAR  STRUCTURE  OF  PLANTS 

The  bodies  of  all  plants  and  animals  have  been  found  to  be 
composed  of  organic  units  termed  cells.  Each  cell  unit  in  its 
simplest  form  is  a  boxlike  structure  with  bounding  cell  walls 
inclosing  the  cell  cavity  in  which  is  lodged  the  living  substance 
of  the  organism.  These  structural  units  of  the  plant  and  animal 
body  were  first  called  cells  on  account  of  their  fancied  resemblance 
to  the  cells  of  a  honeycomb  or  of  a  prison. 

The  lowest  forms  of  plants  and  animals  are  unicellular  in 
structure,  but  the  higher  forms  are  multicellular,  consisting  of 
innumerable  cells  of  the  most  varied  form  and  function.  Thus 
muscles  and  bones,  roots,  stems,  and  leaves,  are  all  built  up  of 
cells,  which  are  adapted  in  their  form  and  structure  to  the 
peculiar  functions  of  each  particular  organ  or  part. 

Just  as  bricks  or  stones  are  used  as  structural  units  with 
which  to  build  a  wall  or  a  tower,  so  we  may  conceive  of  the 
bodies  of  the  higher  animals  and  plants  as  being  built  up  of  liv- 
ing cell  units.  It  has  been  found  also  that  nearly  all  organisms 
have  their  origin  in  a  single  cell,  the  fertilized  egg  cell. 

In  the  building  of  the  complex  organism  from  this  single  egg 
cell  the  egg  first  divides  into  a  multitude  of  simple  cells  like 
itself,  and  these  similar  cells  are  then  gradually  transformed  into 
the  different  kinds  of  cells  which  make  up  the  various  tissues 
and  organs  of  the  adult  body. 

This  conception  that  all  living  organisms,  however  diverse  in 
their  character,  are  composed  of  similar  structural  cell  units  and 
of  a  common  living  substance,  protoplasm,  is  now  known  as  the 

45 


46 


GENERAL   BOTANY 


cell  theory.  It  is  needless  to  say  that  this  generalization  was 
the  result  of  prolonged  scientific  observation  and  research,  ex- 
tending over  many  years  and  engaged  in  by  a  host  of  scientific 
men,  each  of.  whom  added  his  mite  to  the  complete  conception. 


PLANT  CELLS  AND  THEIR  PARTS 

The  cell  structure  of  plants  outlined  above,  and  the  general 
structure  of  the  plant  cell,  can  be  most  easily  demonstrated  by 

the  student  in  thin  sections  cut 
from  the  softer  parts  of  plants 
and  examined  under  a  compound 
microscope. 

If  such  sections  are  cut  from 
dead  tissues  like  bottle  cork  or 
pith,  the  honeycomblike  appear- 
ance of  the  sections  will  be  very 
obvious  (Fig.  26,  a).  The  entire 
section  will  appear,  like  the  honey- 
comb, to  be  made  up  of  boxlike 
units  termed  cells,  each  cell  unit 
being  composed  of  a  cell  wall  and 
a  cell  cavity  inclosed  by  it.  In 

FIG.  26.    Cellular  structure  of  pith      CQrk    tisgue     the    eellg    fit   closely 

without  spaces  between  them, 
exactly  like  honeycomb  cells ;  but 


'ell  cavity 


and  celery 
a,  note  the  intercellular  spaces  and 


thin  cell  walls ;  b,  cells  in  transverse 

section;  c,  same  in  both  transverse      m    pith    and    Various    Other    plant 

section  and  long  section.    The  cell 

cavity  is  shaded  in  c,  and  the  thick-    tissues  the  cells  round  up  as  they 

ened  portions  of  the  cell  wall  are  left  grow    an(J  leaye  spaces,  especially 

light.    The  primary,  or  first,  wall  is  b  .  J 

indicated  hy  a  middle  line.   Consult  at   the   junctions   of   the  Cells  With 

the  text  for  the  method  of  cell-wall  ea(jh  other>     These  spaces  serve  in 
thickening  in  such  a  case  as  this 

many    instances   for   the    passage 

of  air  through  the  plant  and  are  called  intercellular  spaces. 
If  sections  similar  to  the  above  are  cut  from  living  tissues, 
as  from  a  root  (Fig.  27),  the  same  general  cellular  structure 
will  be  observed  as  that  seen  in  sections  of  dead  tissues.  The 
cell  cavities  of  the  living  cells  will,  however,  appear  to  be  filled 


THE  CELLULAR  STRUCTURE  OF  PLANTS 


47 


Cell  WCLll 

• — Cytoplasm 

— -Nucleus 

"    'Vacuole  — 


with  a  semifluid,  viscid  substance,  not  unlike  the  honey  in  a 
honeycomb  in  consistency  and  in  general  appearance.  This  sub- 
stance, which  fills  the  cell  cavities  of  all  living  plant  cells,  is  the 
living  substance  of  the  plant  body,  to  which  Von  Mohl  first  gave 
the  name  protoplasm.  If  sections  of  roots  or  of  other  living 
parts  of  plants  are  properly  stained,  this  living  substance,  proto- 
plasm, within  the  cell  cavities  will  be  found  to  be  composed  of 
a  darker  central  body,  called 
the  nucleus,  and  a  less  dense 
part  surrounding  the  nucleus, 
called  the  cytoplasm.  For 
convenience  in  designating 
the  parts  of  a  living  cell  the 
entire  mass  of  living  proto- 
plasm within  one  cell  cavity 
is  termed  the  protoplast, 
which,  as  we  have  already 
seen,  is  composed  of  two 
distinct  parts,  the  cytoplasm 
and  the  nucleus. 

The     protoplast    in    young 

f         *  /  The  figures  are  designed  to  illustrate  the 

plant  Cells  (a)  usually  bears  a      Cell  parts  and   the  gradual  formation  of 

vacuoles.  a,  a  young  cell  with  small  vacu- 
oles ;  b-d,  progressive  vacuole  formation  as 
it  occurs  during  the  enlargement  of  a  cell 
by  growth.  Consult  the  text  for  further 


'Nucleus — 
—Nucleolus 

Cytoplasmic 
c  d 

FIG.  27.       Camera     drawings,     greatly 

magnified,  of  root  tip  cells  containing 

cytoplasm,  nuclei,  and  vacuoles 


discussion 


somewhat  different  relation 
to  the  other  parts  of  the  cell 
from  what  it  does  in  older 
cells  (b-d).  In  young  cells 
it  fills  the  cell  cavity  with  a  dense  mass  of  protoplasm,  while  in 
the  older  cells  it  contains  water  in  the  form  of  water  drops, 
called  vacuoles.  As  the  cells  grow,  these  water  drops,  or 
vacuoles,  enlarge  and  unite  until  they  finally  accumulate  as  a 
large  central  water  drop,  vacuole,  or  sap  cavity  in  the  center 
of  the  cell.  In  such  instances  the  solid  protoplast  of  the  young 
cell  becomes  gradually  forced  outward  as  the  cell  grows  by  the 
accumulation  of  water  in  the  large  central  vacuole.  The  cyto- 
plasmic  portion  of  the  protoplast  then  forms  a  thin  layer,  lining 
the  cell  wall  and  inclosing  the  central  vacuole.  It  is  now  prop- 
erly called  the  cytoplasmic  sac  (d).  The  nucleus  in  these  older 


48  GENERAL  BOTANY 

cells  with  a  large  central  sap  cavity  is  either  suspended  in  the 
center  of  the  cell  by  fine  cytoplasmic  fibrils,  or  strands,  extend- 
ing from. the  nucleus  to  the  cytoplasmic  sac,  or  it  lies  embedded 
in  the  cytoplasmic  sac  next  to  the  cell  wall.  In  any  case  a  dense 
envelope  of  cytoplasm  surrounds  the  nucleus. 

The  cell  wall  of  all  plant  cells  is  formed  by  the  living  proto- 
plasm, or  protoplast,  which  finally  occupies  the  cell  cavity  sur- 
rounded by  the  secreted  cell  wall.  In  the  development  of  a  plant 
from  a  fertilized  egg  cell  the  first  wall  is  secreted  by  its  proto- 
plast as  a  protective  envelope  for  the  living  protoplasm.  All 
subsequently  formed  walls  are  laid  down  between  the  two  halves 
of  the  protoplast  in  a  dividing  cell.  The^  wall  first  formed  is 
aygjjiin  and  delicate  and  is  composed  of  pectose.  Upon  this 

re  is  immediately  deposited  a  whitish  substance  called  cellu- 
rose,  which  is  closely  related  to  starch  and  sugar  in  its  chemical 
composition.  Bleached  celery  is  composed  largely  of  the  cellulose 
which  forms  the  cell  walls,  and  a  bleached  celery  stalk  will  give 
the  student  a  good  conception  of  the  general  appearance  and 
nature  of  this  cell-wall  substance  of  plant  cells. 

Although  the  first  wall  is  always  thin,  it  may  become  greatly 
changed  in  its  character  as  the  cells  of  the  plant  body  differ- 
entiate to  form  the  plant  organs  and  tissues.  These  modifications 
of  the  cell  wall  are  brought  about  by  two  distinct  methods: 
first,  by  cell-wall  thickening ;  second,  by  changes  in  its  chemical 
character. 

The  thickening  of  the  cell  wall  may  be  effected  by  the  de- 
posit of  new  layers  of  cellulose  on  the  primary  thin  wall  by  the 
cell  protoplast,  in  much  the  same  way  as  a  new  layer  of  plaster 
might  be  added  to  the  wall  of  a  house  ;  or  the  new  wall  substance 
may  filter  into  the  cell  wall  and  be  deposited  between  the  cellulose 
particles  of  the  original  cell  wall.  The  thick-walled  cells  which 
form  the  supporting  layers  of  stems  and  leafstalks  (Fig.  26,  b 
and  <?)  are  illustrations  of  thick-walled  cells  whose  walls  are 
composed  largely  of  cellulose,  while  the  wood  and  bast  fibers 
(Figs.  52  and  53)  from  which  wood  pulp,  linen,  and  hemp  are 
made  are  illustrations  of  thick-walled  cells  in  which  the  chem- 
ical character  of  the  cell- wall  substance  has  been  changed  by 


THE  CELLULAR  STRUCTURE  OF  PLANTS    49 

infiltration  of  lignin  or  a  similar  substance  made  by  the  living 
protoplasts  of  these  cells.  In  these  highly  modified  cells  the  pro- 
toplast usually  disappears  after  the  cell  wall  is  fully  formed,  and 
they  become  dead  cells  in  which  the  cell  cavities  are  filled  with  air. 

The  skeleton,  or  supporting  framework,  of  plants  is  thus  com- 
posed of  the  cell  walls  of  the  individual  cells  making  up  the 
plant  body.  In  the  case  of  the  higher  plants  these  cell  walls  be- 
come thickened  and  hardened,  as  indicated  above,  and  form  a  firm 
supporting  skeleton  for  trees  and  other  plants  of  large  size.  This 
skeleton  of  cell  walls  divides  the  living  substance  (protoplasm) 
of  the  entire  plant  body  into  separate  units,  or  protoplasts,  each 
protoplast  being  surrounded  by  its  own  skeletonlike  cell  wall. 
This  is  quite  unlike  the  condition  found  in  animals,  where  the 
cells  often  have  no  definite  cell  walls  and  are  supported  by  an 
internal  body  skeleton,  as  in  higher  animals,  or  by  a  crustlike 
external  skeleton,  as  in  insects  and  crayfish. 

The  plastids  are  minute  granules  of  denser  protoplasm  which 
occur  in  most  living  plant  cells.  They  occur  in  the  cell  cytoplasm 
and  vary  in  form  from  minute  granules  to  very  elaborate  bands 
and  disks,  such  as  are  found  in  some  algae.  Some  plastids 
are  green,  others  are  composed  of  colorless  cytoplasm,  while 
still  others  are  tinted  with  yellow  and  orange  pigments.  They 
all  agree  in  being  composed  of  living  protoplasm.  They  are 
classified  according  to  their  color  into  chloroplastids,  which  are 
green ;  leucoplastids,  which  are  colorless ;  chromoplastids,  which 
are  variously  tinted. 

Chloroplastids  are  found  in  all  parts  of  plants  which  have  a 
green  color,  such  as  leaves  and  the  outer  parts  of  some  stems. 
The  green  color  in  such  organs  is  due  to  a  green  pigment,  chlo- 
rophyll, which  is  secreted  within  the  numerous  plastids  con- 
tained within  the  cells  of  such  structures  (Fig.  28,  a  and  5). 
Each  chloroplastid  (5)  is  composed  of  a  cytoplasmic  granule,  or 
disk,  and  the  green  pigment  which  gives  it  its  color.  In  such  thin 
leaves  as  those  of  mosses  or  of  the  water  weed  Elodea  the  plas- 
tids are  plainly  visible  under  low  powers  of  the  microscope, 
embedded  in  the  cell  cytoplasm.  In  Elodea  the  plastids  are  often 
carried  around  the  cells  in  the  moving  cytoplasm,  like  boats  in 


50 


GENERAL  BOTANY 


a  stream,  and  serve  to  indicate  the  rate  and  direction  of  cyto- 
plasmic  movement  in  the  leaf  cells  of  this  plant.  The  green 
pigment,  chlorophyll,  can  be  removed  from  the  cytoplasmic 
body  of  the  plastids  by  treating  the  leaves  with  alcohol,  when 
the  green  pigment  dissolves  out,  leaving  the  colorless  plastid 
behind  in  the  cytoplasmic  sac.  We  have  already  learned  that 
the  green  pigment  enables  the  chloroplastids,  with  the  aid  of 
the  sun's  energy,  to  build  and  store  starch  made  from  carbon 
dioxide  and  water.  The  chloroplastids  are  thus  of  the  greatest 
importance  in  making  the  necessary  starchy  food  for  the  plant. 


toplasmic  sac 


Starch 


a       Chloroplastid 


Starch 


Leucoplastid 


iromoplastids 

FIG.  28.   Three  kinds  of  plastids  and  the  formation  of  starch  grains 

a,  a  mature  cell  with  chloroplastids  embedded  in  the  cytoplasmic  sac;  b,  a  single 

chloroplastid  containing  light-colored  starch  grains ;  c,  chromoplastids ;  d,  leucoplas- 

tids  of  begonia  forming  starch  grains 

Leucoplastids  (Fig.  28,  d)  are  similar  to  chloroplastids  except 
that  they  lack  the  green  pigment,  chlorophyll,  and  are  therefore 
unable  to  manufacture  starch  from  raw  food  elements  such  as 
carbon  dioxide  and  water.  They  are  able,  however,  to  transform 
sugar  into  starch,  and  are  therefore  present  in  all  such  special 
storage  organs  as  potato  tubers  and  seeds,  as  well  as  in  ordinary 
stems  and  leaves  where  starch  is  stored  away  from  direct  sun- 
light. They  are  more  difficult  to  demonstrate  microscopically 
than  chloroplastids,  and  hence  are  less  frequently  seen  than  the 
latter.  Leucoplastids  are  often  transformed  into  chloroplastids 
by  the  secretion  of  green  pigments  when  they  are  exposed  to 
sunlight.  This  is  frequently  observed  when  potato  tubers  are 
uncovered  and  turn  green  in  the  part  of  the  tuber  which  is 
directly  exposed  to  the  sun. 


THE  CELLULAR  STRUCTURE  OF  PLANTS    51 

Chromoplastids  (Fig.  28,  c)  are  plastids  which  secrete  pigments 
other  than  green,  which  tint  the  plastids  and  the  organs  in 
which  they  are  located  yellow,  orange,  or  red.  Flowers  and 
fruits  often  owe  their  color  to  the  abundant  chromoplastids  de- 
veloped in  the  cytoplasm  of  their  cells.  Familiar  examples  are 
the  yellow  petals  of  nasturtium  flowers,  the  red  color  of  the 
tomato,  and  the  orange  yellow  of  the  berry  of  bittersweet. 


FUNCTIONS  OF  CELL  PARTS 

The  cell  wall,  as  has  been  stated,  serves  as  a  protection  for 
the  delicate  protoplasts  and  as  a  skeletal  framework  for  the 
entire  plant  body.  The  protoplasts  of  the  plant  body  have  been 
shown  to  be  connected  in  some  plants  by  delicate  strands  of 
protoplasm,  which  pass  through  fine  pores  in  the  separating 
cell  walls.  The  protoplasts  in  these  plants  are  not,  therefore, 
isolated  units  of  living  substances,  but  are  closely  joined  in 
a  living  body  of  protoplasmic  units.  Such  a  continuity  of  the 
protoplasts  by  means  of  connecting  strands  might  conceivably 
be  of  direct  service  in  the  passage  of  foods  from  cell  to  cell  or 
in  establishing  a  nervous  connection  between  the  various  tissues 
and  organs  of  the  plant  body.  Both  of  these  hypotheses  have 
been  advanced  by  reputable  botanists,  but  there  is  as  yet  no 
conclusive  proof  that  either  is  true. 

The  cytoplasm  and  the  nucleus  undoubtedly  represent  a  divi- 
sion of  labor  in  the  living  substance  of  the  protoplast  by  which 
it  is  enabled  to  do  its  complex  work  more  satisfactorily. 

The  cytoplasm  is  supposed  to  be  largely  concerned  with  the 
building,  storage,  and  use  of  foods  and  with  the  reception  of  im- 
pressions (stimuli)  from  the  outside.  We  have  already  seen  that 
the  green  chloroplastids  are  portions  of  the  cytoplasm  which 
build  starch,  and  we  shall  learn  that  the  starch  in  potatoes  and  in 
seeds  is  stored  in  the  cytoplasm  of  the  cells  of  those  structures  by 
the  agency  of  leucoplastids.  It  is  equally  evident  that  the  cyto- 
plasm must  be  the  first  to  feel  impressions  from  outside  of  the  cell. 

The  vacuole  inclosed  by  the  cytoplasmic  sac  is  filled  with  water 
in  which  various  substances  formed  by  the  cytoplasm  finally 


52  GENERAL  BOTANY 

accumulate.  This  water  and  the  substances  —  sugar,  acids,  and 
salts  —  which  diffuse  into  it  from  or  through  the  cytoplasmic 
sac  are  together  called  the  cell  sap.  In  the  cells  of  the  sugar 
beet,  sugar  is  the  main  constituent  of  the  cell  sap,  while  in  fruits 
both  sugar,  acids,  and  salts  are  found  in  considerable  quantities. 
The  cell  sap  thus  becomes  a  storehouse  for  the  water  and  foods 
which  may  be  needed  by  the  cell  and  the  plant. 

The  nucleus  is  also  very  important  in  the  life  of  the  cell  and 
the  organism,  being  very  closely  connected  with  fertilization  and 
with  the  formation  of  new  cells  by  division.  The  most  important 
act  of  fertilization  seems  to  be  the  union  of  the  nucleus  of  the 
male  cell  or  gamete  with  that  of  the  female  cell  or  gamete,  while 
the  complex  changes  undergone  by  the  chromatin  of  the  nucleus 
in  cell  division  indicates  its  importance  in  that  process.  Although 
the  nucleus  and  cytoplasm  are  thus  separated  structurally  and 
seem  to  have  certain  specialized  functions,  it  has  been  conclu- 
sively shown  that  they  are  coworkers  in  the  various  cell  activi- 
ties and  are  mutually  dependent  upon  each  other  for  continued 
existence. 

The  following  table  summarizes  the  relations  of  the  constituents 
of  the  cell  in  terms  of  living  and  lifeless  parts  as  discussed  above. 


SUMMAKY 

1.  Lifeless  parts  of  plant  cells : 

The   cell  wall:    outer   membranous   covering   of    the   living 

protoplast  of  a  cell. 
Vacuoles  and  cell  sap :   the  cavities,  or  spaces  (termed  sap 

cavities)  in  the  protoplasts  of  plant  cells,  and  the  contained 

cell  sap,  composed  of  water  and  other  substances  in  solution 

within  the  vacuole. 
Metaplasmic  bodies :  solid  waste  and  food  particles  embedded 

in  the  protoplast. 

2.  Living  parts  of  plant  cells : 

Protoplast :  the  entire  living  protoplasm  of  the  cell,  including 

the  nucleus. 
Cytoplasm:    all  protoplasm  of  the  cell  exclusive  of  nucleus 

and  nucleolus. 


THE  CELLULAR  STRUCTURE  OF  PLANTS    53 

Cytoplasmic  sac:  the  term  used  to  designate  the  cytoplasm 
of  the  cell  when  it  takes  the  form  of  a  sac  lining  the  cell 
wall  and  bounding  the  central  water  vacuole. 

Nucleus :  a  denser  differentiated  protoplasmic  body  within  the 
cell  bounded  by  the  nuclear  membrane. 

Nudeolus:  a  darker  dense  mass  of  protoplasm  within  the 
nuclear  cavity.  It  is  usually  surrounded  in  plant  cells  by 
a  nuclear  vacuole. 

Plastids :  differentiated  grains  or  bodies  of  cytoplasm  of 
different  color  and  function. 


CHAPTER  IV 

HISTORICAL  SKETCH  (THE  CELL  AND  THE  CELL  THEORY) 
EARLY  DISCOVERIES  AND  THEORIES 

Cell  wall.  The  first  discovery  of  the  cell  structure  of  plants 
is  attributed  to  Robert  Hooke,  an  Englishman,  who,  about  the 
year  1665,  observed  the  cellular  structure  of  plant  tissue  in 
sections  of  the  wood,  bark,  and  leaves  of  plants.  Hooke  was 
impressed  with  the  fact  that  such  sections,  when  viewed  under 
a  strong  microscope,  presented  the  same  appearance  as  the  cells 
of  a  honeycomb.  He  therefore  applied  the  term  cell  to  the 
cavity  which  he  saw  inclosed  by  the  conspicuous .  cell  walls, 
which  form  the  cell  boundary  of  most  plant  cells.  This  appli- 
cation of  the  word  cell  to  the  cavity,  and  not  to  its  contents, 
is  now  regarded  as  a  misnomer,  since  we  know  that  the  most 
important  part  of  plant  and  animal  cells  is  the  living  substance, 
protoplasm,  which  is  contained  in  the  cell  cavity.  Long  usage 
has  so  firmly  established  the  term,  however,  that  it  is  still  in  use, 
although  its  meaning  is  now  extended  to  include,  with  the  cell 
wall,  the  substances  within  the  cell  cavity,  the  most  important 
of  which  is  the  living  protoplasm. 

The  protoplasm  and  nucleus.  The  observation  of  the  living 
substance  within  the  cell  cavities  of  plant  and  animal  cells, 
and  the  recognition  of  its  true  nature  and  significance,  was  not 
understood  for  fully  two  centuries  after  Hooke  made  his  dis- 
covery of  the  cellular  structure  of  plants.  This  was  due  in 
part  to  faulty  observations  and  in  part  to  the  imperfect 
microscopes  with  which  the  observations  were  made. 

Robert  Brown  (1831),  an  English  botanist,  discovered  a 
dark  central  body  within  the  cells  of  orchids,  which  he  named 
the  cell  nucleus.  Other  observers  of  cellular  structure  saw  a 
substance  outside  of  the  cell  nucleus  in  plant  cells,  which 

54 


THE  CELL  AND  THE  CELL  THEORY       55 

they  termed  variously  gum,  mucilage,  and  sarcode.  The  real 
nature  of  this  substance  within  the  cell  wall  was  not,  however, 
positively  known  until  about  the  middle  of  the  last  century, 
when  its  true  significance  as  the  essential  living  substance  of 
the  cell  and  of  the  organism  was  recognized.  The  name  proto- 
plasm, which  it  now  bears,  was  first  given  to  this  living  sub- 
stance in  animal  cells  by  Purkinje  (1839-1840)  and  in  plant 
cells  by  Von  Mohl  (1846).  Von  Mohl's  conception  of  the 
nature  of  the  living  substance  of  plant  cells,  including  the 
nucleus,  is  embodied  in  the  following  quotation,  taken  from 
his  work  on  "  The  Vegetable  Cell." 

In  the  center  of  the  young  cell,  with  rare  exceptions,  lies  the 
so-called  nucleus  cellulae  of  Robert  Brown.  It  is  usually  of  very 
considerable  size  in  proportion  to  the  magnitude  of  the  young 
cell,  so  that  in  particular  cases  —  for  example,  in  the  cells  of  jointed 
hairs  —  it  almost  fills  the  cavity.  The  remainder  of  the  cell  is  more 
or  less  densely  filled  with  an  opaque,  viscid  fluid  of  a  white  color, 
having  granules  mingled  in  it,  which  fluid  I  call  protoplasm. 

Von  Mohl  and  his  contemporaries,  Schleiden  and  Nageli, 
not  only  clarified  the  current  conceptions  of  the  nature  of  the 
cell  and  of  protoplasm  but  also  established  the  fact  of  cell 
differentiation,  or  change  in  the  form  and  structure  of  cells  to 
serve  different  functions  in  the  plant.  They  discovered  that  the 
stems,  roots,  and  leaves  of  plants  are  composed  of  a  great 
variety  of  cells,  varying  in  form,  structure,  and  function,  which 
enable  the  plant  organism  to  do  its  work  more  effectively  than 
it  otherwise  could. 

The  cell  theory.  The  idea  that  all  living  plants  and  animals, 
however  diverse  in  their  nature,  are  composed  of  similar  cell 
units  was  first  published  by  Schleiden,  a  botanist,  and  Schwann, 
a  zoologist,  in  1839.  Later  investigations  have  not  only  cor- 
roborated the  truth  of  this  generalization  but  have  greatly 
extended  its  meaning  and  emphasized  its  importance.  It  has 
been  found  also  not  only  that  animals  and  plants  are  similar 
in  general  cellular  structure  but  that  the  living  substance  is 
not  essentially  different  in  the  two  kingdoms  of  life.  The 


56  GENERAL  BOTANY 

latter  idea  was  first  formally  established  and  published  by 
Max  Schultze  in  1863.  This  essential  identity  in  cellular 
structure,  and  in  the  nature  of  the  living  substance  of  all 
organisms,  explains  the  great  similarity  which  has  long  been 
known  to  exist  between  the  lowest  plants  and  animals.  Indeed, 
this  similarity  is  so  great  that  certain  organisms  are  to-day 
claimed  by  botanists  as  belonging  to  the  plant  kingdom  and  by 
zoologists  as  belonging  to  the  animal  kingdom.  The  great 
importance  of  the  cell  theory  can  only  be  realized  by  the  stu- 
dent as  we  proceed  to  study  more  intimately  the  biology  of  the 
higher  and  the  lower  plants,  in  which  we  find  exactly  the  same 
processes  performed  by  very  different  structures  and  organs  of  the 
plant  body. 

LATER  DISCOVERIES  AND  THEORIES 

The  later  developments  of  the  cell  theory  were  the  result 
of  researches  and  discoveries  made  by  a  notable  group  of 
biologists  during  the  latter  part  of  the  nineteenth  century. 
These  later  observers  applied  the  early  ideas  and  discoveries 
concerning  the  structure  of  the  cell  and  protoplasm  to  prob- 
lems concerned  with  the  ultimate  structure  of  protoplasm  and 
nucleus,  cell  divisions,  fertilization,  and  development,  and  to 
the  fundamental  processes  of  plant  and  animal  physiology. 
Their  discoveries  and  conclusions  in  these  various  fields  can 
only  be  outlined  here,  but  the  following  brief  survey  is  given  in 
order  that  the  student  may  realize,  in  part  at  least,  the  full 
significance  of  the  cell  theory  to  modern  biological  thought 
and  discovery. 

Cell  division.  The  general  process  of  cell  division  in  the 
higher  plants  was  first  correctly  interpreted  by  Karl  Nageli 
in  1846,  although  Schleiden  and  Von  Mohl  made  large  con- 
tributions to  the  final  solution  of  the  problem. 

Basing  their  observations  on  those  of  Nageli,  Von  Mohl,  and 
Schleiden,  the  later  botanists  of  the  nineteenth  century,  under 
the  leadership  of  Eduard  Strasburger,  the  great  German  botanist, 
worked  out  in  much  greater  detail  the  mechanism  and  the  mean- 
ing of  the  division  process  now  designated  as  mitosis. 


THE  CELL  AND  THE  CELL  THEORY 


57 


Fertilization.  The  process  of  fertilization  was  first  observed 
in  Spirogyra,  one  of  the  lower  water  plants,  in  1879,  and  in  the 
higher  plants  by  Strasburger  in  1884. 

The  essential  act  of  this  fertilizing  process  was  found  to  con- 
sist in  the  union  of  two  cells,  male  and  female,  termed  gametes  or, 
more  frequently,  sperm  cell  and  egg  cell  (Fig.  29).  Later  studies 
seem  to  indicate  that  the  union  of  the  male  and  female  nuclei  is 
the  most  important  if  not  the  only  essential  part  of  the  process. 


Embryo 


Fertilization    Zygote  Cell  division   Cell  di/erentiatioj^ 
Gametes 


Organ  formation 

FIG.  29.   A  diagram  illustrating  the  main  cell  processes  occurring  during  the 
development  of  a  plant 

Consult  the  text  for  further  discussion 

The  two  gamete  cells,  when  united  in  fertilization,  form  a 
new  cell,  the  zygote  cell,  which  is  a  double  cell  in  the  sense  that 
it  is  composed  of  two  cells  from  two  distinct  individual  plants 
or  from  separate  portions  of  one  plant.  This  double  nature  of 
the  zygote  cell  will  be  found,  in  our  later  studies  in  reproduc- 
tion, to  be  of  great  significance  in  heredity,  since  it  contains  the 
fundaments  of  two  sets  of  characters,  one  set  brought  in  by  the 
male  gamete  and  one  by  the  female  gamete. 

Embryology.  The  development  of  a  higher  plant  from  the 
zygote  was  first  completely  worked  out  by  Hanstein  in  1869. 
The  development  process  of  the  higher  plant,  like  that  of  a 
higher  animal,  was  found  to  embrace  three  important  stages,  or 
phases,  each  phase  including  processes  of  cell  division  and 
growth.  The  first  stage,  or  phase,  is  concerned  with  the  division 
of  the  single-celled  zygote  into  many  cells,  thus  giving  rise  to 


58  GENERAL  BOTANY 

a  cellular  embryo.  In  the  second  phase  of  development  the 
uniform  cells  of  the  early  cellular  embryo  begin  to  differentiate 
to  form  the  first  organs  and  tissues  of  the  early  embryo.  In  the 
third  phase  this  early  embryo  forms  the  seedling,  in  which  the 
parts  outlined  in  the  early  embryo  take  on  the  form  of  the  parts 
of  the  mature  plant. 

The  mature  form  and  structure  of  the  plant  is  derived  from 
the  seedling  by  the  growth  and  differentiation  of  cells  located 
in  .particular  growing  regions,  called  meristems  and  cambiums, 
at  the  tips  of  the  stem,  roots,  and  branches  and  in  the  internal 
growing  layers  (cambiums)  of  the  plant  body. 

Physiology.  The  physiology  of  both  plants  and  animals  has 
been  shown  to  be  dependent  upon  the  physiology  of  the  con- 
stituent cells  of  the  plant  and  animal  body.  Nerve  cells  and 
their  processes  are  known  to  receive  and  send  impulses  which 
cause  muscle  cells  to  contract  and  move  the  parts  of  the  an- 
imal body.  In  a  similar  manner  the  cells  of  plants  cause  rapid 
or  slow  movements  in  roots,  stems,  and  leaves.  In  the  sensitive 
plant  (Mimosa)  the  movements  caused  by  specialized  cells  at  the 
base  of  the  leaves  and  leaflets  are  as  rapid  as  those  of  some 
animals,  but  the  slower  movements  of  the  parts  of  plants  have 
been  shown  to  be  no  less  cellular  in  their  mechanism  than  those 
of  Mimosa  and  in  animals.  Similarly,  food-building  and  the  cir- 
culation and  use  of  foods  are  ultimately  traceable  to  the  activi- 
ties of  the  constituent  cells  of  plants  and  animals.  In  the  higher 
plants  food-building  takes  place  in  the  green  leaf  cells  provided 
with  chloroplasts ;  circulation  of  foods,  water,  and  salts  occurs 
in  long  chains  of  cells  (ducts  and  sieve  tubes);  while  assimila- 
tion goes  on  in  all  living  cells  of  the  plant  body.  In  the  light 
of  these  facts  the  organism  is  seen  to  be  a  great  and  highly  dif- 
ferentiated colony  of  cells,  which,  like  the  members  of  a  civi- 
lized community  or  of  a  factory,  are  organized  to  maintain  the 
welfare  of  the  community  as  well  as  that  of  each  individual 
member.  In  the  studies  which  follow,  the  student  will  be  in- 
terested in  gaining  at  first  hand  a  knowledge  of  the  cell 
structure  and  cell  activities  of  plants  upon  which  the  above 
generalizations  of  the  modern  cell  theory  are  based. 


THE  CELL  AND  THE  CELL  THEORY  59 

SUMMARY 

1.  The  cell  discovered  and  named  as  a  unit  of  organic  structure 

in  plants  (Robert  Hooke,  1665). 

2.  The  nucleus  discovered  in  orchidaceous  plants  (Robert  Brown, 

1831). 

3.  The  term  protoplasm  applied  to  living  substance  of  plant  cells 

(Von  Mohl,  1846). 

4.  Cell  theory  announced  (Schleiden  and  Schwann,  1839). 

5.  Cell  division   correctly  interpreted   (Nageli,  Von  Mohl,  and 

Schleiden,  about  1846). 

6.  Cell  differentiation  and  tissue  structure  explained  (Von  Mohl 

and  contemporaries,  1840-1850). 

7.  Later  developments  of  the  cell  theory  : 

a.  Fertilization  in  Spirogyra  (1879) ;  in  higher  plants  (Stras- 

burger,  1884). 

b.  Development  of  plant  embryo  (Hanstein,  1869). 

c.  Cytology    (1873-1900).     Minute    structure    of    cytoplasm 

and  nucleus ;  details  of  nuclear  and  cell  division.    Stras- 
burger  the  leading  botanist  of  this  period  in  cytology. 

d.  Physiology  of  plants  (1860-1900).    Sachs  and  Pfeffer  the 

most  prominent  plant  physiologists. 

The  above  summary  indicates  the  general  course  of  the  in- 
vestigations and  discoveries  which  have  established  the  cell 
theory  and  have  furnished  us  with  our  present  knowledge  of 
the  structure  and  development  of  organisms.  The  student  who 
has  followed  this  development  of  the  cell  theory  will  be  better 
fitted  to  understand  the  bearing  of  the  more  detailed  account 
of  the  cell  structure,  development,  and  activities  of  plants  which 
now  follows  in  the  text. 


CHAPTER  V 


GROWTH  AND  CELL  DIVISION 

The  growth  of  the  higher  plants  is  very  different  from  that  of 
the  higher  animals.  In  animals  growth  ceases  after  a  relatively 
short  period  in  the  life  of  the  organism ;  plants,  on  the  contrary, 
continue  to  grow  throughout  their  whole 
life,  which,  as  in  the  case  of  long-lived 
trees,  may  extend  over  hundreds  of  years. 
In  the  early  stages  of  the  growth  of  a 
higher  plant  from  the  egg  cell  all  parts 
of  the  embryo  grow  until  the  seedling 
is  thoroughly  established  for  self-support. 
In  annual  and  biennial  plants  growth 
may  continue  to  be  general  over  a  large 
part  of  the  plant  body,  although  it  usually 
becomes  localized  as  maturity  approaches. 
In  perennials,  however,  including  our 
common  trees  and  shrubs,  growth  and  its 
accompanying  cell  division  is  largely  lim- 
ited to  the  tips  of  the  roots  and  shoots 
and  to  the  cambiums,  or  growing  cell 
layers,  which  produce  annually  new  lay- 
ers of  wood  and  bark.  Buds  are  familiar 
examples  of  this  localization  of  growth 


Lateral  branch^.. 

A 


FIG.  30.  Growing  areas 
of  a  buckwheat  plant. 
The  growing  parts  are 
stippled;  the  permanent 
portions  are  white 


at  the  branch  tips,  and  we  shall  find  a  similar  provision  at  the 
tips  of  roots.  Fig.  30  illustrates  diagrammatically  the  growing 
zones  of  an  ordinary  herbaceous  plant  in  which  growth  has 
become  localized. 

Since  growth  is  one  of  the  most  important  phenomena  in  the 
life  of  plants,  we  can  profitably  devote  a  considerable  space  to  an 
account  of  a  concrete  case  of  cell  and  organ  growth,  which  will 
furnish  a  foundation  for  understanding  all  growth  more  clearly. 

60 


GEOWTH  AND  CELL  DIVISION 


61 


For  this  the  root  tip  furnishes  the  best  material  both  for  obser- 
vations of  growth  in  a  living  object  and  for  the  study  of  its  more 
detailed  features  as  revealed  in  its  cellular  organization. 


Roolcap-* 


Permanent 
Maturing 

Elongating 
Meristem 


GROWTH  AND  STRUCTURE  OF  THE  ROOT 

The  elongation  of  the  root  is  easily  observed  in  living  roots 
by  selecting  the  first  or  primary  root,  which  springs  from  a  seed 
during  its  early  stages  'of  germination.  In  order  to  locate  the 
exact  region  in  which  growth  in 
length  takes  place  the  tip  of  the 
root  should  first  be  marked  with 
fine  lines,  evenly  spaced,  for 
several  millimeters  back  of  the 
apex  of  the  root  tip.  In  Fig.  31 
these  lines  have  been  made  at 
intervals  of  two  millimeters  in  6, 
and  the  separation  of  the  lines 
in  c  indicates  where  the  root 
grew  in  length  for  the  first 
twenty-four  hours  after  it  was 
marked.  It  will  be  noted  by 
consulting  the  figures  that  the  root  elongated  in  the  second  and 
third  millimeter  spaces  back  of  its  apex,  and  that  the  first  milli- 
meter space  at  the  very  apex  of  the  root  tip  and  the  millimeter 
spaces  just  back  of  the  elongating  area  have  remained  unchanged. 
In  the  first  millimeter  or  two,  at  the  apex  of  the  root,  two  dis- 
tinct groups  of  cells  exist,  with  which  we  shall  be  concerned  as 
we  proceed  to  study  the  method  of  growth  in  the  root  tip.  One 
of  these  is  apparent  in  living  roots  as  a  light-colored,  conical 
mass  of  cells  constituting  the  apical  point  of  the  root.  This 
light-colored  point  is  the  protective  rootcap,  which  incloses  a 
disk  of  cells,  not  more  than  a  millimeter  in  length,  which  is 
invisible  in  fresh  roots  but  is  easily  recognized  in  sections  of 
growing  root  tips.  This  latter  disk  of  cells  is  the  meristem,  or 
dividing  cell  zone,  which  plays  a  very  prominent  part  in  root 
growth,  since  it  furnishes  most  of  the  new  cells  for  the  rootcap 


FIG.  31.    Growth  in  length  of  the 
root  of  corn 

The  separation  of  the  millimeter  spaces 
indicates  the  place  of  greatest  elongation 


62 


GENERAL  BOTANY 


and  elongating  zones.  For  our  convenience  in  describing  the 
growth  of  the  root  the  tip  may  therefore  be  divided  (Fig.  31,  c) 
into  the  following  cell  zones :  the  rootcap,  the  meristem  (just 
above  the  rootcap  and  inclosed  by  it),  and  the  elongating,  matur- 
ing, and  permanent  zones.  The  term  permanent  zone,  as  here 
used,  signifies  all  that  portion  of  the  root  tip  in  which  the  cells 
have  ceased  to  grow.  In  the  lower  portion  of  this  zone  the  cells 

have  not  yet  become 
fully  differentiated 
into  the  different 
tissues  of  the  mature 
root. 

Structure.  Before 
proceeding  with  the 
more  detailed  aspects 
of  the  growth  of  the 
root  tip  it  will  be 
necessary  to  consider 
the  cellular  organi- 
zation of  the  root  at 
its  apex.  For  this 
purpose  thin  sections 
which  have  been  es- 
pecially prepared  for 
microscopic  study  are 
necessary.  First  the 
roots  are  embedded  in 
paraffin  wax ;  then  the  sections  are  cut  lengthwise  of  the  roots 
with  special  section-cutting  machines,  or  microtomes.  The  sec- 
tions thus  obtained  are  fastened  on  glass  slides  and  are  stained 
by  special  methods  for  microscopic  observation.  In  sections  so 
prepared  the  student  can  easily  determine  the  main  cell  areas  of 
the  growing  root  tip  which  have  already  been  observed  in  living 
roots,  and  can  also  relate  the  cell  structure  of  these  areas  to  the 
processes  of  cell  and  organ  growth. 

The  cellular  structure  of  the  root  tip  is  illustrated  in  Fig.  32,  a 
and  6,  which  are  microphotographs  of  a  portion  of  a  root  tip  and 


FIG.  32.    Microphotographs  illustrating  the  cellular 
structure  of  the  root  tip 

a,  a  long  section  through  a  growing  root  tip  of 
the  false  Solomon's  seal  (Smilacina)  (the  lighter 
cells  of  the  rootcap,  the  darker  meristem  zone,  and 
the  longer  cells  of  the  elongating  region  are  plainly 
shown) ;  b,  cells  of  a  portion  of  the  elongating  zone, 
greatly  magnified 


GEOWTH  AND  CELL  DIVISION 


63 


C 


of  some  of  its  cells  highly  magnified.  Fig.  33  is  a  diagrammatic 
drawing  of  a  median  long  section  of  a  root  in  connection  with 
which  are  shown  camera  drawings  of  separate  groups  of  cells 
taken  from  the  rootcap,  the  meristem,  the  elongating  zone,  the 
maturing  zone,  and 
the  permanent  zone. 
These  latter  drawings 
not  only  indicate  the 
actual  form  of  the  cells 
of  the  above  regions 
of  the  root  but,  since 
they  are  all  drawn  to 
the  same  scale,  they 
show  also  the  increase 
in  size  of  the  root- 
tip  cells  from  the 
meristem  to  the  per- 
manent zone,  where 
they  cease  to  grow  in 
length. 

The  rootcap  is  a 
conical  mass  of  light- 
colored  cells  that  ter- 
minates the  root  tip 
and  forms  a  protective 

mantle  for  the  actively  FIG.  33.    The  growth  of  cells  in  a  root  tip 

dividing  and  growing  «>  the  main  areas  of  the  root  tip  shaded ;  b-e,  camera 

.                    ni          i  •   r  drawings  of  cells  from  the  various  regions  of  the  root 

meristem    Cells    Which  as  indicated  in  the  figure.    Note  the  changes  in  form 

lie  just  back  of  it.    The  and  size  of  the  cells. as  growth  proceeds.   The  nuclei 

„     ,  grow  with  the  cells,  but  they  occupy  a  proportionately 

Outer  cells  OI  the  cap  smaller  part  of  the  cell  cavity  as  the  cells  enlarge 

are  large,  thin-walled, 

and  loosely  joined  together.  Their  lighter  color  is  due  to 
the  large  vacuoles  which  nearly  fill  the  cell  cavity  of  each  cell. 
These  outer  cells  are  being  constantly  worn  off  as  the  tip  of 
the  root  is  forced  through  the  soil  by  the  elongation  of  por- 
tions of  the  root  above  or  behind  the  cap  cells.  The  inner  cells 
merge  insensibly  into  the  cells  of  the  meristem  above  them  and, 


64  GENEBAL  BOTANY 

like  the  latter,  have  the  power  of  active  cell  division,  thus  pro- 
ducing new  cells  to  replace  those  worn  off  at  the  apex  by  contact 
with  the  particles  of  soil.  There  is  no  danger,  therefore,  that  the 
root  will  be  deprived  of  its  protective  cap,  even  though  it  is  con- 
stantly losing  its  outer  cells  by  death  and  abrasion  from  the  soil. 

The  meristem  layer  is  one  or  two  millimeters  in  length  and  is 
composed  of  a  mass  of  cells  which  fit  into  the  upper,  concave  sur- 
face of  the  rootcap.  Its  cells  are  more  or  less  cuboidal  in  form, 
with  dense  cytoplasm  and  large  nuclei.  The  meristem  layer  is 
therefore  readily  recognized  in  stained  preparations,  both  by  the 
small  size  and  regular  form  of  its  cells  and  by  its  darker  color 
(Fig.  32,  a),  due  to  the  activity  with  which  the  dense  protoplasts 
of  the  meristem  cells  take  up  and  retain  artificial  stains.  The 
nuclei  are  also  large  in  proportion  to  the  size  of  the  cells  and 
often  show  clearly  the  heavily  stained  chromatin  which  makes 
up  the  bulk  of  the  nuclear  protoplasm.  Cell  division  is  not  so 
frequently  observed  in  the  meristem  cells  as  in  those  of  the 
elongating  zone,  although  this  process  is  one  of  the  distinguish- 
ing characteristics  of  the  meristem.  The  reason  for  this  seems 
to  be  the  extremely  short  period  of  time  required  by  these  cells 
to  complete  the  complicated  process  of  cell  and  nuclear  division. 

The  elongating  zone 1  occupies  about  four  millimeters  of  the 
length  of  the  root  immediately  back  of  the  meristem  zone. 
Its  cells  are  distinguished  from  those  of  the  meristem  region 
by  their  lighter  color  in  stained  preparations  and  by  their 
greater  average  length.  The  lighter  color  of  its  cells  is  due 
to  their  less  dense  cytoplasm  as  compared  with  that  of  the 
meristem  cells,  and  to  the  gradual  accumulation  in  their  proto- 
plasts of  absorbed  water  in  the  form  of  vacuoles,  which  increase 
in  number  and  size  toward  the  upper  limit  of  the  elongating 
zone.  We  consequently  find  that  the  cells  in  the  lower  half 
of  the  elongating  zone  resemble  closely  those  of  the  meristem, 
into  which  they  graduate  insensibly  at  the  junction  of  the  two 
cell  areas,  while  the  cells  of  the  upper  half  are  longer  and 
lighter-colored,  with  large  vacuoles. 

1  The  upper  part  of  the  elongating  zone,  in  which  the  cells  are  beginning  to 
differentiate  into  permanent  tissue,  is  here  designated  as  maturing. 


GROWTH  AND  CELL  DIVISION 


65 


The  permanent  zone  merges  gradually  and  indistinguishably 
into  the  elongating  zone  below  it  and  into  the  mature  portions 
of  the  root  immediately  above  it.  In  general,  however,  its  cells 
are  characterized  by  their  great  length  and  large  central 
vacuoles,  or  sap  cavities.  The  cyptoplasm  in  the  longer  cells 
is  usually  in  the  form  of  a  delicate  cyptoplasmic  sac,  in  which 
the  nucleus  is  embedded.  The 
nuclei  appear  to  be  rela- 
tively smaller  in  these  cells, 
but  accurate  measurements 
indicate  that  they  usually 
maintain  a  slow  growth  as 
long  as  the  cells  increase  in 
size,  and  that  they  do  not  di- 
minish in  volume  in  the  cells 
in  which  growth  is  completed. 

Elongation.  In  a  growing 
root  the  meristem  layer  gives 
rise  to  hundreds  of  new  cells. 
As  these  new  cells  are  formed 
part  of  them  remain  meristem 
cells,  while  others  in  the  upper 
portion  of  the  meristem  begin 
to  elongate  and  form  a  part 
of  the  elongating  zone,  as 
shown  in  Fig.  34.  At  the 


FIG.  34.   A  diagram  illustrating  the 
method  of  elongation  of  a  root 

a,  shows  the  main  zones  of  the  root  shaded 
as  in  Fig.  33,  a ;  in  6,  the  same  zones  are 
shown  after  the  root  has  elongated.  Note 
that  the  meristem  and  elongating  zones 
remain  of  the  same  length  in  a  and  6  as 
growth  proceeds 


same  time  the  cells  in  the 
upper  part  of  the  elongating 
zone  are  reaching  their  definite  size  and  will  add  a  new  disk 
of  cells  to  the  length  of  the  permanent  zone  of  the  root.  It 
will  therefore  be  clear  that  by  this  process  the  permanent  part 
of  the  root  becomes  continually  longer  by  successive  additions 
from  the  elongating  zone,  while  the  length  of  the  latter  zone 
is  kept  constant  by  a  similar  number  of  cells  derived  from  the 
division  of  the  meristem  cells.  By  these  two  processes  —  namely, 
cell  division  and  cell  elongation — the  root  thus  grows  in  length 
and  its  tip  advances  through  the  soil. 


66 


GENERAL  BOTANY 


7. 


GROWTH  OF  OTHER  PLANT   ORGANS 

The  same  phenomena  which  we  have  outlined  in  the  growth 
of  the  root  obtain  in  the  growth  of  all  other  plant  organs, 
including  leaves,  buds,  and  stems. 

Cell  division  and  cell  enlargement  are  the  two  main  phenomena 
of  all  growth  which  are  demonstrable  by  scientific  methods.  In 
the  case  of  plants  the  enlargement  of  cells  in  growth  seems  to 

be  mainly  due  to  the  absorption 
of  water  and  the  inflation  of  the 
cells  by  osmotic  pressure.  By  this 
means  very  rapid  growth  can  take 
place  without  the  expenditure  of 
the  energy  necessary  to  manufac- 
ture great  quantities  of  living  sub- 
stance with  which  to  fill  the  cell 
cavities  of  the  expanding  cells.  In 
the  slower-growing  animals  the 
water-inflation  method  of  cell 
growth  does  not  exist  to  any  such 
extent  as  it  does  in  plants,  since 
the  latter  need  to  adapt  them- 
selves quickly  to  the  changing 
seasons,  which  necessitates  the 
rapid  production  of  extensive  root 
and  leaf  surfaces  for  the  absorption  of  water  and  gases  at  the 
opening  of  each  warm  season.  The  growth  of  leaves  in  spring 
is  a  good  illustration  of  this  aspect  of  growth  in  plant  organs. 
A  large  tree  will  produce  in  a  few  days  in  spring  many 
square  yards  of  leaf  surface ;  this  involves  the  production  and 
inflation  of  thousands  of  new  cells.  It  is  obvious  that  the  tree 
would  be  wholly  unable  in  this  short  time  to  manufacture 
enough  new  protoplasm  to  fill  the  tremendous  space  caused 
by  the  expansion  of  the  cell  cavities  of  its  leaf  cells.  The 
water-inflation  method  is  therefore  both  economical  and  neces- 
sary in  all  rapidly  growing  plant  organs,  such  as  leaves,  stems, 
and  roots. 


FIG.  35.    Growth  of  leaves  of 
the  lilac 

The  spread  of  the  squares  indicates 

a  uniform  growth  of  the  leaf  over  its 

entire  surface 


GROWTH  AND  CELL  DIVISION 


67 


The  place  where  growth  takes  place  varies  in  different  organs. 
In  most  leaves  (Fig.  35)  growth  is  uniform  over  the  entire  leaf 
surface,  although  in  the  case  of  long,  narrow  leaves  it  may  con- 
tinue longer  at  the  base  than  at  the  apex.  In  stems  growth  is 
more  localized  than  in  leaves ;  it  continues  for  a  longer  period 
and  extends  over  a  larger  portion  of  the  stem  than  it  does  in  the 
root  tip  just  studied.  In 
herbaceous  stems  (Fig.  36) 
growth  may  continue  at 
the  apex  of  the  main 
shoot  or  its  branches  for 
several  weeks  or  for  the 
entire  growing  season,  as 
in  the  root.  This  growth, 
like  that  of  the  root,  is 
due  to  an  active  meristem 
at  the  stem  tip,  within 
the  apical  buds,  in  which 
active  cell  division  fur- 
nishes new  cells  for  con- 
tinuous growth  in  the 
elongating  zone,  which, 
as  indicated  above,  may 
extend  over  several-inter- 
nodes  at  the  apex  of  the 
stem. 


FIG.  36.    Growth  in  length  of  a  root  of  corn 
and  of  the  stem  of  a  bean  seedling; 


The  spread  of  the  markings  in  b  and  d  indicates 

the   places    of   greatest   growth.     Observe    the 

greater  area  over  which  growth  takes  place  in 

the  stem  as  compared  with  the  root 


In  woody  stems  growth 
in  length  takes  place  in 
terminal  or  in  lateral  buds,  as  in  herbaceous  stems.  The  buds 
of  woody  stems,  however,  usually  contain  most  or  all  of  the 
parts  of  the  next  season's  shoot  formed  in  advance,  so  that  its 
growth  in  the  spring  is  completed  more  quickly  than  in  herba- 
ceous stems,  and  the  plant  is  thus  able  to  expand  its  leaves  in 
a  comparatively  short  time  when  conditions  favorable  to  spring 
growth  occur.  In  the  lilac  (Fig.  37)  the  bud  a  is  composed  of 
the  leaves  of  the  next  season,  arranged  on  the  very  short  axis  of 
the  future  branch.  The  internodes  separating  the  successive 


68 


GENERAL  BOTANY 


-teams 


-Bud  scales  - 


pairs  of  leaves  at  the  nodes  are  very  short,  as  shown  in  the  long 
section  of  the  bud  b.  The  meristem  terminates  the  axis  of  the 
bud,  all  of  which  will  constitute  the  elongating  zone  when 
growth  first  starts  in  the  spring.  The  expansion  of  the  inter- 
nodes  by  growth  separates  the  leaves  (c  and  d),  which  at  first 
grow  more  rapidly  on  their  inner  than  on  their  outer  surfaces, 

and  so  unfold  and  spread 
out  horizontally  to  the  light. 
As  growth  proceeds  the  cells 
within  the  lower  internodes 
reach  their  definite  length  and 
thus  add  new  segments  to  the 
permanent  portions  of  the 
young  stem,  as  in  the  case 
of  roots.  In  this  permanent 
portion  the  cells  differentiate 
and  the  main  tissues  of  the 
mature  stem  are  formed.  Be- 
fore growth  in  length  ceases, 
the  buds  for  the  next  season 
become  apparent  and  proceed 
to  form  the  young  leaves  for 
the  growth  of  the  following 
spring.  Indeed,  the  growing 
points,  or  meristems,  for  these 
buds  are  apparent,  in  the 
lilac,  in  the  wintering  bud  in 
the  axils  of  its  rudimentary  leaves.  In  the  autumn,  therefore, 
when  the  leaves  fall,  the  lilac  shoot  of  the  season  presents  the 
aspect  shown  in  Fig.  38,  5,  with  all  the  buds  laid  down  and 
protected  by  bud  scales. 

In  all  these  changes  incident  to  growth  the  cells  and  organs 
of  plants  are  subject  to  the  effect  of  environmental  forces. 
A  certain  amount  of  water  and  heat,  varying  with  the  type 
of  organism,  is  necessary  for  growth.  Light  usually  has  a 
retarding  effect  on  growth,  although  this  too  is  a  variable 
factor  in  its  influence.  In  general  it  may  be  said  that  extremes 


—Leaves — , 


ud  scales 


FIG.  37.   Bud  structure  and  growth  in 
the  lilac 

a,  6,  surface  and  sectional  views  of  a  bud 

in  the  resting  condition ;  c,  d,  similar  views 

of  the  same  bud  in   spring  when  it  first 

begins  to  grow 


GROWTH  AND  CELL  DIVISION 


69 


of  heat,  cold,  moisture,  and  food  supply  are  detrimental  to 
growth,  while  mean  conditions  in  the  case  of  any  given  factor 
are  likely  to  be  favorable  to  it. 


SUMMARY 

Growth  in  plants  comprises  three  main  phases  or  stages.  The  first 
is  that  of  cell  division,  in  which  the  cells  of  the  entire  structure,  or 
of  specialized  parts  termed  meristems  or  cambiums,  undergo  rapid 
multiplication  by  mitosis. 
The  second  phase  is  that 
of  enlargement,  during 
which  the  new  cells 
formed  by  mitosis  in- 
crease greatly  in  size, 
and  so  cause  the  growth 
of  the  organ  or  part  con- 
cerned. The  third  is  the 
phase  of  differentiation 
and  maturation,  in  which 
the  enlarged  cells  are 
modified  to  form  the  dif- 
ferent tissues  of  the  per- 
manent plant  organs. 

Plants  differ  from 
animals  principally  in 
the  second  and  third 
phases  of  both  cell  and 
organ  growth.  Plant  cells 
increase  in  size  very 
largely  by  the  absorp- 
tion of  water,  which  collects  inside  the  cells  in  the  form  of  water 
drops,  or  vacuoles.  In  animals  the  increase  in  the  size  of  the  cells  is 
largely  due  to  actual  increase  in  the  amount  of  living  substance. 
The  differentiation  and  maturation  stages  of  cell  growth  are  also 
necessarily  different  in  plant  and  animal  cells,  on  account  of  the  dif- 
ferent kinds  of  permanent  tissue  formed  in  plant  and  animal  organs. 

Plant  organs  differ  in  their  methods  of  growth  according  to  the 
nature  of  the  organ  or  part  and  the  length  of  time  during  which 
enlargement  continues.  Organs  which  have  a  short  period  of  growth, 


FIG.  38.   Lilac  twigs  in  summer  and  winter 
conditions 

and  b  represent  the  season's  growth  of  the  bud 
shown  in  Fig.  37 


70 


GENERAL  BOTANY 


like  leaves,  fruits,  seeds,  and  tubers,  usually  grow  more  like  animals, 
—  by  a  uniform  increase  in  the  size  of  cells  throughout  the  entire 
structure.  Cylindrical  organs,  on  the  contrary,  like  roots  and  stems, 
which  have  periods  of  enlargement  often  extending  over  many 
years,  have  definitely  located  growing  masses,  or  layers,  of  cells,  called 
meristems  and  cambiums.  These  meristem  and  cambium  cells  renew 
their  cell  divisions  each  season,  in  the  case  of  perennial  organs, 

and  contribute  new 
cells,  which  then  pass 
through  the  second 
and  third  phases  of 
growth  to  form  new 
tissues  and  organs. 
The  apical  meristems 
which  terminate  the 
roots,  the  stems,  and 
the  branches  have  al- 
ready been  studied. 
The  cambiums  are 
cylindrical  layers  of 
meristem  cells  lying 
between  the  wood  and 
the  bark  in  steins  and 
roots  and  between  the 
outer  cork  and  inner 
bark.  These  cam- 
biums enable  roots 


FIG.  39.    Diagrams  illustrating  the  method  of  elon- 
gation of  roots  and  herbaceous  stems 


a,  6,  elongation  of  the  root  tip  illustrated  as  in  Fig.  34; 

c,  (/,  similar  method  of  representing  the  elongation  of  a 

herbaceous  stem  like  the  bean  (Fig.  36).   Consult  the 

text  for  further  discussion 


and    stems    to    grow 
in  diameter  and  also 

to  form  new  annual  tissue  layers  for  protection  and  for  the  con- 
duction and  storage  of  foods,  water,  and  soil  salts.  The  apical 
meristems  also  enable  plants  to  send  out  new  leaves  and  roots  each 
season  for  the  absorption  of  raw  food  elements  and  the  compounding 
of  these *raw  materials  into  organic  foods. 

The  above  general  statements  relative  to  the  growth  of  stems  and 
roots  by  means  of  apical  meristems  and  cambium  layers  will  be  more 
readily  understood  by  reference  to  the  following  diagrammatic  figures: 
Fig.  39,  a  and  b,  is  a  repetition  of  Fig.  34,  illustrating  the  method 
of  growth  of  roots  in  length  by  means  of  an  apical  meristem.  Fig.  39, 
c  and  d,  illustrates  in  a  similar  manner  the  growth  of  a  herbaceous 


GROWTH  AND  CELL  DIVISION 


71 


stem,  represented  in  Fig.  36,  e  and  d.  In  the  latter  cases  the  meri- 
stem  is  practically  identical  in  form  and  structure  with  that  of  the 
root  tip.  It  does  not,  however,  produce  a  protective  rootcap,  since 
it  is  here  protected  by  the  enveloping  leaves  of  the  terminal  bud. 

Growth  in  length  takes  place  in  the  stem,  as  in  the  root,  by  the 
continuous  transformation  of  cells  produced  by  the  division  of  tl  o 
meristem  cells  into  cells  of  the  elongat- 
ing zone.    Later  these  become  permanent 
tissue  and  form  the  mature  nodal  and 
internodal  tissue  of  the  older  portions  of 
the  stem.   At  the  same  time  a  cylindrical 
cambium  layer,  in  the  region  indicated  by 
the  dark  lines  in  the   figure,  begins  to 
divide  to  form  the  new  water-conducting 
and  food-conducting  tissue 
of  the  vascular,  or  woody, 
cylinder  of  the  plant. 

Fig.  40  represents  the 
elongation  of  a  woody  stem 
by  a  large  terminal  bud. 
In  this  instance  the  gen- 
eral structure  of  the  meri- 
stem and  the  method  of 
elongation  are  the  same  as 
in  the  herbaceous  stem. 
The  differences  between 
the  two  arise  from  the 
fact  that  the  herbaceous 
stem  has  a  more  continu- 
ous growth  throughout  the 
season  than  the  woody 

stem,  and  from  the  fact  that  the  buds  of  the  herbaceous  stems  con- 
tain only  a  small  part  of  the  season's  growth  in  miniature,  while 
those  of  the  woody  stem  contain  all  of  the  nodes,  internodes,  and 
leaves  for  the  next  year,  laid  down  in  advance.  When  such  a  bud 
(a)  begins  to  grow  in  the  spring,  the  tissues  of  the  elongating  zone, 
laid  down  the  year  before,  produce  the  terminal  twig  of  the  sea- 
son (c),  while  the  meristem  produces  a  new  bud,  shown  in  c,  resem- 
bling the  mother  bud  (a)  in  structure  and  function.  The  tissues  of 
the  elongating  zone  in  a  are  already  partially  differentiated,  but 


FIG.  40.    Three  diagrams  showing  method 
of  elongation  of  a  lilac  bud 

a,  bud  in  winter  condition,  similar  to  Fig.  37,  b ; 

b,  elongation  stage  in  spring,  similar  to  Fig.  37,  d; 

c,  permanent  condition,  similar  to  Fig.  38.   Meri- 
stem, elongating,  and  permanent  zones  shaded  as 

in  Figs.  33,  34,  and  39 


72  GENERAL  BOTANY 

with  their  great  increase  in  length,  b  and  c,  they  gradually  form 
the  tissues  of  the  bark,  cortex,  and  woody  cylinder  of  the  fully 
formed  twig  (c).  While  this  extension  and  differentiation  is  pro- 
ceeding, a  new  cylindrical  cambium,  formed  within  the  wood 
cylinder,  adds  new  conducting  tissue  to  the  stem  in  a  manner 
to  be  explained  later. 

From  the  above  discussion  it  is  apparent  that  roots  and  stems 
grow  in  essentially  the  same  manner,  with  slight  modifications  due 
to  the  nature  of  the  particular  organ  and  its  environment. 

Growth  also  plays  a  prominent  part  in  the  movements  of  plant 
organs  and  in  the  ultimate  form  of  the  plant  body,  as  shown  in  an 
earlier  chapter. 

THE  CELL  AND  CELL  DIVISION 
MINUTE  STRUCTURE  OF  THE  CELL 

Before  proceeding  to  discuss  the  complicated  process  of  cell 
division  it  will  be  necessary  to  review  briefly  the  structure  of 
the  cell  and  to  point  out  certain  details  of  cell 
structure  which  we  have  not  heretofore  consid- 
ered. In  the  brief  discussion  which  follows,  the 
term  resting  cell  will  be  used  to  indicate  the  con- 
dition of  a  cell  when  it  is  neither  dividing  nor 
in  active  preparation  for  cell  division.  Fig.  41 
is  a  camera  drawing  of  a  resting  cell  of  a  root 

"  .     .  tip,  in  which  it  may  be  seen  that  the  protoplasm 

a   root  tip   in  the         f>  j  r 

resting  stage  of  such  a  cell  presents  the  aspect  of  a  mesh,  or 
Note  the  distributed  network,  inclosing  light  spaces.  In  the  living 
condition  of  the  dark  state  faQ  meshwork  would  be  made  up  of  liv- 

chromatin  net  . 

ing  cytoplasm,  and  the  light  spaces  would 
correspond  to  minute  vacuoles.  This  appearance  of  the  living 
substance  in  artificial  preparations  has  received  various  interpre- 
tations, which  cannot  be  discussed  in  an  elementary  textbook. 

The  simplest  conception  for  the  beginning  student  is  to  regard 
the  living  substance  as  spongy  in  structure,  resembling  the  struc- 
ture of  a  fine  bath  sponge,  with  lamellse,  or  plates,  of  living  pro- 
toplasm surrounding  minute  spaces  filled  with  cell  sap.  Such  a 
spongework,  when  cut  in  thin  sections,  would  appear  like  the 


GBOWTH  AND  CELL  DIVISION  73 

network  of  the  cell  protoplasm  in  our  figure.  The  nuclear  wall, 
or  membrane,  is  a  delicate  bounding  membrane  which  in  the  liv- 
ing condition  of  the  cell  is  composed  of  cytoplasm.  Within  this 
membrane  the  nuclear  protoplasm  is  seen  to  have  essentially  the 
same  structure  as  the  cytoplasm,  except  that  the  bounding  walls 
of  the  meshes  in  the  nuclear  protoplasm  are  heavier  and  more 
granular  and  stain  more  deeply  in  permanent  preparations.  This 
deeply  staining  protoplasm  of  the  nucleus  is  called  chromatin 
(from  chroma,  meaning  "  color  ")  on  account  of  its  avidity  for 
various  stains  used  in  preparing  cells  for  microscopic  study. 
The  chromatin  substance  is  very  essential  in  cell  division  and 
in  reproduction,  and  its  distribution  in  the  nucleus  of  the  resting 
cell  is  thus  of  extreme  importance.  In  addition  to  the  chromatin 
network  the  figure  shows  a  conspicuous  nucleolus  surrounded  by 
a  characteristic  nuclear  vacuole.  Under  high  powers  of  the  micro- 
scope the  nucleolus  appears  to  be  suspended  in  this  vacuole  by 
delicate  strands  of  nuclear  substance  which  connect  the  nucleolus 
with  the  chromatin  network,  or  spongework.  With  this  prelimi- 
nary review  of  the  minute  structure  of  a  resting  cell  we  may 
proceed  to  consider  the  stages  of  cell  division. 

CELL  DIVISION  —  MITOSIS 

Function.  We  have  learned  that  the  cells  in  the  meristem  .and 
the  elongating  zones  of  growing  root  and  stem  tips  increase  in 
number  by  cell  division  and  that  the  cells  thus  produced  grow 
into  the  mature  organs  and  tissue  of  the  plant.  The  same  thing 
happens  also  when  the  single  egg  cell  of  the  plant  forms  a  many- 
celled  embryo  by  division.  The  embryo  then  differentiates  and 
grows  into  a  new  organism.  The  multiplying,  or  increase  in 
number,  of  cells  in  an  organism  for  purposes  of  growth  is  there- 
fore the  most  obvious  use  of  cell  division  in  plants. 

Although  this  production  of  new  cells  appears  at  first  sight 
to  be  the  main  object  of  mitosis,  biologists  in  recent  times  have 
called  attention  to  the  remarkable  precision  with  which  the  chro- 
matin substance  of  the  nucleus  is  divided  and  to  the  complicated 
mechanisms  of  mitosis  by  which  this  equal  division  is  effected. 


74  GENERAL  BOTANY 

The  goal  of  cell  division,  therefore,  is  probably  not  simply  a 
division  of  a  mother  cell  into  two  essentially  equal  daughter 
cells  but  also  the  distribution  of  exactly  equivalent  masses  of 
chromatin,  called  chromosomes,  to  each  daughter  nucleus  of  the 
newly  formed  cells.  We  shall  learn  that  this  equal  division  and 
distribution  of  the  chromatin  substance  to  all  the  cells  of  a  com- 
plex organism  is  the  basis  for  the  modern  theories  concerning 
reproduction  and  inheritance.  We  may  therefore  anticipate  with 
interest  the  discussion  of  these  important  and  complicated  proc- 
esses, concerned  with  the  division  of  the  cell  and  the  nucleus, 
which  are  comprehended  under  the  general  term  mitosis. 

Process.  It  is  customary,  for  the  sake  of  clearness,  to  describe 
the  various  processes  of  mitosis  under  certain  phases  or  stages 
(Fig.  42).  These  phases,  taken  in  the  order  of  their  occurrence, 
are  the  prophase,  metaphase,  anaphase,  and  telophase.  The  stu- 
dent should  bear  in  mind,  however,  that  the  processes  included 
under  the  above  phases  are  continuous  processes,  and  that  one 
phase  graduates  insensibly  into  the  next  phase,  which  follows  it 
in  orderly  sequence.  Some  of  these  phases,  such  as  the  meta- 
phase, are  undoubtedly  longer  than  others,  which  are  passed 
through  more  quickly.  This  is  indicated  by  the  fact  that  in 
prepared  slides  certain  phases  are  much  more  prominent  than 
others,  for  the  probable  reason  that  their  longer  duration  makes 
it  easier  to  fix  a  larger  number  of  nuclei  in  these  stages.  All 
the  phases  of  mitosis  are  passed  through  rapidly,  however,  and 
the  entire  process  of  cell  and  nuclear  division  never  occupies 
more  than  a  few  hours. 

The  prophase  is  the  preparatory  phase  of  nuclear  division, 
during  which  changes  take  place  in  the  nucleus  and  the  cell 
preparatory  to  the  equal  division  of  the  chromatin  substance, 
which  is  the  principal  goal  of  nuclear  division.  These  prepara- 
tory changes  involve  two  structures  which  play  an  important 
part  in  the  ultimate  division  of  the  nucleus.  These  structures 
are  the  chromatin,  which  we  have  learned  is  a  permanent  portion 
of  the  nucleus,  and  the  spindle,  which  is  a  temporary  structure 
apparently  designed  as  a  framework  on  which  the  chromatin 
gathers  and  finally  becomes  distributed  in  equal  amounts  to  the 


GROWTH  AND  CELL  DIVISION 


75 


daughter  nuclei.  Although  the  changes  which  the  chromatin 
undergoes  during  prophase  are  coincident  with  the  building  of 
the  spindle,  it  is  easier  and  clearer  to  discuss  the  chromatin 
changes  and  the  origin  of  the  spindle  separately.  We  are  already 


FIG.  42.   Drawings  illustrating  the  main  stages  in  mitosis 

a-e,  prophase ;  /,  metaphase ;  g,  anaphase ;  h-i,  telophase ;  j-k,  cell  division ; 
I,  daughter  nuclei 

familiar  with  the  fact  that  the  chromatin  in  the  nucleus  of  a  rest- 
ing cell  is  in  the  form  of  a  net,  or  meshwork.  In  early  prophase, 
when  the  nucleus  begins  its  preparation  for  division,  it  may  be 
noted  that  the  chromatin  granules  begin  to  accumulate  along 
definite  portions  of  the  resting  network,  thus  giving  rise  to  denser 
masses  of  chromatin  at  certain  points  (a).  These  dense  masses 
of  chromatin,  which  are  at  first  irregular,  ultimately  assume  a 


76  GENERAL  BOTANY 

more  definite  form  and  arrangement  within  the  nuclear  cavity. 
In  some  instances  they  seem  to  form  a  continuous  band  of  chro- 
matin  substance,  which  is  called  the  spirerne  (ft).  In  other  cases 
definite  rodlike  masses,  called  chromosomes  (c),  emerge  directly 
from  the  nuclear  network.  In  either  case  the  conversion  of  the 
network  of  the  resting  nucleus  into  the  definite  masses  known 
as  chromosomes  is  most  easily  understood  by  supposing  that 
after  the  chromatin  granules  have  condensed  to  form  either  the 
spireme  or  the  isolated  chromosomes,  the  remaining  portions  of 
the  original  network  break  down  and  leave  the  chromosomes  free. 

As  soon  as  the  chromosomes  are  formed  in  the  above  manner 
they  shorten  and  thicken  and  finally  take  up  a  definite  position 
at  the  periphery  of  the  nuclear  cavity  next  to  the  nuclear 
membrane.  The  chromatin  rods  are  now  ready  for  the  equal 
division  of  their  substance  preparatory  to  the  formation  of 
daughter  nuclei.  Since  the  further  changes  in  the  chromo- 
somes during  prophase  are  concerned  with  the  spindle,  they 
will  be  described  in  connection  with  the  following  discussion 
of  that  structure. 

The  spindle  which  is  concerned  with  the  equal  distribution  of 
the  chromatin  substance  in  nuclear  division  may  for  convenience 
be  termed  the  first  spindle,  to  distinguish  it  from  the  second  spin- 
dle, which  is  concerned  with  the  building  of  the  cell  wall  which 
divides  the  cell  as  a  whole.  In  such  cells  as  those  found  in  grow- 
ing root  tips  the  first  spindle  makes  its  appearance  as  two  fibrous 
masses  of  cytoplasm  at  the  opposite  ends,  or  poles,  of  a  nucleus  in 
prophase  (c).  The  fibers,  which  are  composed  of  living  cytoplasm, 
become  more  conspicuous  as  the  spindle  continues  its  forma- 
tion at  either  pole  of  the  nucleus.  Finally  two  half  spindles  are 
formed,  each  with  a  conical  apex  and  a  broad  base.  The  base  of 
each  half  spindle  fits  over  one  pole  of  the  nucleus  like  a  skull- 
cap, and  the  fibers  composing  these  half  spindles  appear  to  extend 
from  its  apex,  or  pole,  to  the  nuclear  membrane  (c?).  When  the 
half  spindles  are  fully  formed,  the  nuclear  membrane  gradually 
disappears,  beginning  at  the  poles  of  the  nucleus,  and  the  half 
spindles  elongate  across  the  nuclear  cavity.  They  ultimately 
unite  to  form  a  complete  single  spindle,  with  a  bulging  equatorial 


GROWTH  AND  CELL  DIVISION  77 

region  and  conical  poles  (e).  The  spindle  fibers  are  supposed 
finally  to  pass  through  the  nuclear  cavity  from  pole  to  pole  of  the 
complete  spindle.  When  the  spindle  began  to  form,  the  chromo- 
somes occupied  the  periphery  of  the  nuclear  cavity  next  to  the 
nuclear  membrane.  As  the  half  spindles  elongate  across  the 
nuclear  cavity  with  the  disappearance  of  the  nuclear  membrane 
the  chromosomes  appear  to  be  pushed  into  the  equatorial  region  of 
the  nucleus  between  the  two  half  spindles,  forming  what  is  some- 
times called  the  equatorial  plate.  The  chromosomes  of  this  equa- 
torial plate  ultimately  move  toward  the  outside  of  the  completed 
spindle  and  become  arranged  in  a  definite  radiate  manner  at  its 
periphery,  attached  to  the  outer  spindle  fibers.  On  account  of 
their  radiate  appearance  at  this  stage  it  is  often  called  the  mother 
star  stage  (/).  The  particular  fibers  to  which  the  chromosomes 
are  attached  are  called  the  traction  fibers,  since,  as  we  shall  learn, 
they  appear  to  contract  and  separate  the  two  halves  of  each 
chromosome  in  the  next  stage  of  mitosis.  The  remaining  fibers 
of  the  spindle  are  termed  the  central  spindle  fibers,  or  supporting 
fibers.  With  the  completion  of  the  spindle  and  the  arrangement 
of  the  chromosomes  in  the  mother  star  stage  the  events  of  the  pro- 
phase  are  completed  and  those  of  the  metaphase_are  ushered  in. 

The  metaphase  stage  (/)  is  concerned  with  the  equal  longitu- 
dinal division  of  the  chromosomes  into  half  chromosomes  and  with 
the  separation  of  these  half  chromosomes  preparatory  to  the  mi- 
gration of  the  chromosomes  to  the  poles  of  the  spindle  to  form  the 
daughter  nuclei.  In  this  process  each  chromosome  splits  through- 
out its  entire  length  into  two  equal  halves,  each  of  which  appears 
to  be  attached  to  a  traction  spindle  fiber.  The  half  chromosomes 
then  begin  to  separate,  as  though  pulled  apart  by  the  shortening 
traction  fibers.  The  final  separation  of  the  half  chromosomes 
marks  the  end  of  the  metaphase  stage. 

The  anaphase  is  the  stage  (g)  in  which  the  half  chromosomes 
continue  their  migration  toward  the  poles  of  the  spindle,  where 
they  finally  arrange  themselves  in  a  more  or  less  radiate  manner 
resembling  somewhat  the  mother  star  stage  of  metaphase.  This 
radiate  arrangement  of  the  daughter  half  chromosomes  is  there- 
fore called  the  daughter  star  stage  and  marks  the  end  of  anaphase 


78  GENERAL  BOTANY 

and  the  beginning  of  telophase  (7i).  During  their  migration  to  the 
poles  the  chromosomes  assume  various  forms  in  the  nuclei  of 
different  plants  and  in  those  of  the  same  plant  in  different  kinds 
of  cells.  In  the  root  tip  the  chromosomes  during  anaphase  are 
usually  greatly  elongated  and  often  hooked  like  a  shepherd's 
staff,  while  in  the  germ  cells  of  the  same  plant  they  are  more 
often  V-shaped  and  greatly  shortened  and  thickened.  In  nuclei 
in  root  tips  the  traction  fibers  appear  to  be  attached  to  the  bent 
chromosomes  at  the  curve  of  the  hooked  chromosomes,  while  in 
the  germ  cells  they  are  attached  at  the  point  of  the  V.  It  should 
perhaps  be  stated  that  there  is  no  evidence  that  the  traction  fibers 
actually  contract  and  pull  the  chromosomes  to  the  poles,  beyond 
the  facts  of  their  apparent  attachment  and  the  peculiar  appear- 
ance of  the  chromosomes  at  this  period,  when  they  look  like 
plastic  rods  being  pulled  poleward.  It  is  quite  possible  that  the 
chromosomes  move  to  the  poles  by  virtue  of  their  own  inherent 
power  of  movement,  or  else  by  attraction  exerted  at  the  poles 
during  anaphase. 

The  telophase  (A,  i)  is  the  final  phase  of  mitosis  and  includes  the 
organization  of  the  daughter  nuclei  and  the  division  of  the  cell 
into  two  daughter  cells.  The  formation  of  the  daughter  nuclei 
occurs  after  the  daughter  star  stage,  which  marks  the  close  of 
anaphase.  The  daughter  chromosomes,  in  the  stage  immediately 
following  the  daughter  star  arrangement,  draw  together  and 
adhere  to  form  a  dense  mass  of  chromatin  at  either  pole  of  the 
nucleus.  A  new  nuclear  membrane  is  now  formed  around  each 
chromatin  mass  by  the  cytoplasm.  Within  each  daughter  nucleus 
thus  initiated  a  nuclear  vacuole  arises ;  the  chromatin  mass  be- 
gins to  loosen  up,  and  the  outlines  of  the  daughter  chromosomes 
reappear.  The  chromosomes  then  begin  to  spread  out  and  unite 
by  new  anastomosing  branches  of  their  chromatin  substance,  while 
vacuoles  appear  in  increasing  number  within  each  chromosome. 
By  the  formation  of  these  vacuoles  and  new  anastomosing  branches 
the  chromosomes  are  soon  reduced  to  the  form  of  a  network,  or 
meshwork,  quite  similar  to  that  of  an  ordinary  resting  nucleus. 
With  the  growth  of  the  chromatin  net  the  nuclear  sap  increases 
in  volume  and  apparently  inflates  the  nuclear  membrane,  which 


GROWTH  AND  CELL  DIVISION  79 

.thus  augments  the  size  of  the  nuclear  cavity.  In  the  nuclei  of 
root  tips  and  similar  vegetative  plant  parts  the  two  daughter 
nuclei  finally  reach  the  stage  of  perfect  resting  nuclei,  with 
chromatin  net,  nuclear  sap,  and  nucleolus.  The  nucleolus  grows 
gradually  with  the  chromatin  net,  but  its  origin  is  still  obscure. 
It  should  not  be  overlooked  by  the  student  that  the  above  proc- 
esses, which  lead  to  the  formation  of  the  new  chromatin  net  of 
the  daughter  nuclei  by  vacuolization  and  anastomosis  of  the 
daughter  chromosomes,  is  exactly  the  reverse  of  the  processes  by 
which  the  chromatin  net  of  a  resting  nucleus  is  transformed  into 
chromosomes.  Condensation  of  a  chromatin  net  to  form  the 
chromosomes  of  the  mother  nucleus  is  always  followed  by  expan- 
sion of  the  chromosomes  to  form  the  resting  net  of  the  daughter 
nuclei  in  the  vegetative  cells  of  plant  organs.  In  germ  cells  a 
slightly  different  procedure  is  usually  manifested  at  certain 
stages  in  their  division  processes. 

O  A 

Cell  division  is  initiated  by  the  formation  of  a  dense  spindle  (/) 
which  we  have  called  the  second  spindle,  in  the  space  occupied 
by  the  central  spindle  fibers  of  the  first  spindle  during  anaphase 
and  early  telophase.  It  soon  becomes  barrel-shaped  (&),  with 
very  dense  outer  peripheral  fibers,  which  stain  heavily  with 
cytoplasmic  stains.  This  second  spindle  unites  the  forming 
daughter  nuclei,  and  has  for  its  function  the  formation  of  the 
new  cellulose  wall,  which  completes  the  separation  of  daughter 
nuclei  and  the  protoplast  of  the  mother  cell  into  two  daughter 
cells.  The  new  separating  cellulose  wall  is  secreted  by  a  dense 
cell  plate  of  cytoplasm,  which  apparently  arises  as  thickenings 
of  the  fibers  of  the  second  spindle  at  the  center  of  each  fiber. 
These  fiber  thickenings  increase  in  size  and  finally  unite  to 
form  a  solid  cytoplasmic  disk,  or  plate  (/),  extending  across  the 
equator  of  the  spindle.  The  spindle  at  the  same  time  increases 
in  diameter  and  stretches  entirely  across  the  cell  from  wall  to 
wall,  dividing  its  protoplast  into  two  equal  parts.  The  cell 
plate  then  splits,  the  split  beginning  at  its  center  and  extending 
to  the  junction  of  the  cell  plate  with  the  cellulose  walls  of  the 
mother  cell.  The  new  cell  wall  is  formed  by  the  deposit  or 
secretion  of  cellulose  particles  between  the  halves  of  the  cell 


80 


GENERAL  BOTANY 


plate.  When  the  wall  is  complete,  the  halves  of  the  cell  plate 
form  the  outer  layer  of  cytoplasm  of  the  newly  formed  daughter 
cells,  next  to  the  new  cell  wall.  Meanwhile  the  meshed  struc- 
ture of  the  cytoplasm  appears  between  each  daughter  nucleus 
and  the  new  cellulose  wall,  which  marks  the  completion  of 
cell  division  and  the  formation  of  the  two  daughter  cells. 
When  one  considers  that  all  of  the  above  phases  of  mitosis 
are  necessary  for  the  formation  of  each  pair  of  new  cells  in  a 
growing  organism,  some  conception  is  gained  of  the  immense 
constructive  activity  going  on  in  every  growing  plant  or  animal. 

REDUCTION  DIVISION  AND  REPRODUCTION 

The  method  of  cell  division,  which  we  have  traced  above  in 
the  cells  of  root  tips,  obtains  in  all  vegetative  parts  of  plants, 
including  roots,  stems,  and  leaves,  and  is  hence  called  vegetative 


FIG.  43.    Diagrams  designed  to  show  the  difference  between  vegetative  and 
reduction  (heterotypical)  mitosis 

«,  ordinary  vegetative  mitosis  in  root-tip  cells  with  equal  division  of  chromosomes, 

as  in  Fig.  43 ;  b,  mitosis  with  reduction  of  chromosomes  to  one  half  in  each  daughter 

nucleus.   Further  discussion  in  the  text 

cell  division  (Fig.  43,  a).  In  this  type  of  mitosis  the  chromosomes 
which  are  formed  in  a  cell  in  prophase  (7)  line  up  on  the  equa- 
tor in  metaphase  (2)  and  split  longitudinally  to  form  daughter 
chromosomes.  These  daughter  chromosomes  then  migrate  to  the 
poles  of  the  spindle  (&)  and  form  the  daughter  nuclei  of  two 
new  daughter  cells  (4).  By  this  method  of  mitosis  all  cells  of 
the  vegetative  plant  body  are  supplied  with  an  equal  number 
of  chromosomes. 


GROWTH  AND  CELL  DIVISION 


81 


In  all  of  the  higher  plants  a'  different  kind  of  cell  division, 
called  the  reduction  division  (Fig.  43,  6),  occurs  in  the  spore 
mother  cells  which  give  rise  to  the  spores  from  which  the 
gametes,  egg  and  sperm  cells,  are  ultimately  derived.  In  such  a 
spore  mother  cell  — •-  for  example,  the  mother  cell  of  a  pollen  grain 
— the  chromosomes  become  associated  in  pairs  in  prophase  (7) 
and  are  thus  arranged  on  the  equator  of  the  spindle  as  double 
chromosomes  in  metaphase  (2).  These  paired  chromosomes  do 
not  split  in  metaphase  as  they  do  in  vegetative  mitosis,  but  the 


Egg  cell 


cell 


Seedling  plant 

FIG.  44.    A  diagram  illustrating  the  history  of  the  chromosomes  in  the  develop- 
ment of  a  plant 

Note  the  following  important  facts  brought  out  in  the  diagram:  The  chromosomes 

are  three  in  number  in  each  gamete ;  they  are  doubled  in  the  zygote  by  fertilization ; 

in  the  succeeding  vegetative  mitoses  the  number  of  chromosomes  remains  the  same 

as  in  the  zygote  cell.   Consult  the  text  for  a  discussion  of  this  figure 

two  chromosomes  of  each  pair  separate  as  whole  chromosomes  and 
migrate  to  opposite  poles  of  the  spindle  in  anaphase  (3)  to  form 
the  chromosomes  of  the  new  daughter  nuclei  (4*).  As  a  result  each 
daughter  nucleus  receives  one  half  of  the  number  of  chromosomes 
contained  in  the  original  mother  cell,  and  so  one  half  of  the  num- 
ber contained  in  all  of  the  vegetative  cells  of  the  plant  body. 

This  method  of  cell  division  is  termed  reduction  division,  and 
it  is  of  the  greatest  importance  in  reproduction  and  in  the  life  his- 
tory of  plants.  In  the  formation  of  spores  in  the  higher  plants 
the  daughter  nuclei  divide  at  once,  by  the  ordinary  vegetative 
method  without  reduction  (^),  to  form  four  nuclei,  which  remain 
associated  in  groups  of  four  called  tetrads  (#).  Each  member 


82  GENERAL  BOTANY 

of  the  tetrad  then  becomes  a  spore,  from  which  the  gamete 
cells,  egg  and  sperm,  are  ultimately  derived.  These  spores  and 
the  gametes  derived  from  them  will  consequently  have  the 
reduced  number  of  chromosomes  characteristic  of  the  reproduc- 
tive cells  of  the  higher  plants. 

Fig.  44  illustrates  the  relation  of  the  reduction  division  to 
sexual  reproduction  and  the  development  of  a  plant  organism. 
In  the  figure  the  male  and  female  gametes,  which  have  been 
derived  from  the  spores  with  the  reduced  number  of  chromo- 
somes, are  represented  as  having  three  chromosomes  each. 
When  these  unite  in  fertilization,  a  zygote  cell  is  formed 
which  contains  six  chromosomes,  or  the  sum  of  the  chromo- 
somes of  the  male  and  female  gametes.  The  divisions  of  the 
zygote  cell  which  follow  in  the  development  of  the  embryo 
and  seedling  are  of  the  vegetative  type  represented  in  Fig.  43,  a, 
and  hence  each  cell  of  the  seedling  will  be  furnished  with  six 
chromosomes.  This  number  remains  constant  also  in  all  the 
vegetative  cell  divisions  which  occur  during  the  development 
of  the  adult  plant. 

It  is  now  known  that  every  species  among  the  higher  plants 
has  the  same  chromosome  history  as  that  sketched  above,  in 
that  the  chromosomes  are  always  doubled  when  fertilization 
occurs  and  are  reduced  to  one  half  the  vegetative  number  in 
the  spores  and  the  gametes.  A  similar  reduction  division  occurs 
in  animals  during  the  formation  of  the  egg  and  sperm  cells.  It 
is  probable  also  that  all  of  the  lower  plants  have  a  reduction 
division  at  some  point  in  their  life  history  which  corresponds  to 
that  described  above  for  the  higher  plants.  It  is  easily  seen  that 
if  reduction  in  the  number  of  chromosomes  did  not  occur  at  some 
point  in  the  life  cycle  of  each  individual  organism,  the  chromo- 
somes would  ultimately  become  innumerable  in  the  cells  of  all  of 
the  higher  plants.  Reduction  division  is  also  supposed  to  have 
an  important  bearing  on  the  method  of  inheritance  of  parental 
characters,  which  will  be  discussed  in  a  later  chapter.  The  great 
precision  and  regularity  with  which  the  vegetative  and  reduction 
cell  divisions  are  carried  out  in  the  life  of  each  organism  is  a 
sufficient  guaranty  of  their  fundamental  biological  importance. 


CHAPTER  VI 

THE  STRUCTURE  AND  FUNCTIONS  OF  STEMS,   ROOTS, 
AND  LEAVES 

WOODY  STEMS 
GROSS  STRUCTURE 

The  most  evident  function  of  the  stem  is  that  of  displaying 
advantageously  the  leaves,  flowers,  and  fruit  for  the  performance 
of  their  proper  functions.  The  main  stem  is  also  an  intermediary 
between  the  roots  and  the  leaves,  and  as  such  it  performs  impor- 
tant functions  in  the  storage  of  reserve  foods  and  in  the  trans- 
portation of  water  and  soluble  food  materials. 

We  shall  soon  learn  that  in  its  external  features  and  in  its 
internal  structure  the  stem  is  a  living,  active  organ  in  which 
structure  and  function  are  admirably  correlated.  In  our  study 
of  the  woody  stem  we  shall  consider  first  its  gross  external 
features  and  then  its  more  minute  internal  structure.  It  may 
interest  the  student  to  know  also  that  woody  stemmed  plants 
are  now  regarded  by  some  botanists  as  the  forerunners  of  the 
soft-stemmed,  herbaceous  plants  of  the  present  day.  It  is  there- 
fore appropriate  to  reverse  the  usual  order  of  presentation  and 
consider  the  woody  stem  first. 

External  features.  Fig.  45  represents  the  external  features  of 
a  shoot  of  a  lilac  which  has  been  produced  by  a  season's  growth, 
as  described  in  the  last  chapter.  In  the  specimen  selected  for 
the  illustration,  both  the  main  shoot  and  the  smaller  lateral 
shoot  have  two  lateral  buds  which  have  replaced  the  terminal 
bud.  This  latter  condition  is  the  more  common  one  in  the  lilac, 
except  in  the  sucker  shoots  which  spring  directly  from  the  roots 
of  the  old  plants.  In  the  figure  it  may  be  seen  that  the  fallen 
leaves  have  left  definite  scars,  the  leaf  scars,  below  each  lateral 

'    83 


84 


GENERAL  BOTANY 


lateral  bud*, 
at  apex  of  stem 


bud,  and  that  the  lower  limit  of  the  season's  growth  on  both  the 

main  and  the  lateral  shoots  is  marked  by  a  ring  of  bud-scale  scars. 

The  brown  bark  which  gradually  covers  over  the  early  green 

bark  of  the  growing  shoot  is  also  seen  to  be  broken  by  minute 

pores,  the  lenticels,  through  which 
air  penetrates  from  the  outside  to 
the  internal  tissues  of  the  shoot. 
Carbon  dioxide  and  water  vapor  are 
also  eliminated  from  the  lenticels  as 
they  are  from  the  stomata  of  leaves. 
These  same  structures,  together  with 
the  wood  cylinder,  will  appear  in  the 
sections  of  the  stem  to  which  we  will 
now  turn  our  attention. 

Internal  structure.  The  gross 
structure  of  the  shoot  is  represented 
in  Fig.  46  as  it  appears  in  a 
transverse  section.  The  epi- 
dermis and  the  brown  bark 
are  not  distinguishable  in 
gross  sections  and  appear 
as  a  single  brown  layer  cov- 
ering the  outside  of  the 
entire  section.  This  layer 
is  designated  as  the  brown 
bark,  within  which  is  the 
green  bark,  composed  of  cells 
which  contain  green  chloro- 
phyll ;  within  the  green  bark 
there  is  an  inner  lighter 
layer,  called  the  inner  bark,  or  phloem.  The  wood  cylinder  is 
clearly  marked,  but  the  cambium,  or  growing  layer,  cannot  be 
clearly  distinguished  from  the  inner  lighter  bark.  Its  position 
at  the  junction  between  the  bark  and  the  wood  is  indicated  by 
a  line.  The  pith  occupies  the  center  of  the  section.  Within 
the  wood  cylinder  two  annual  rings  of  wood  are  represented  as 
having  been  formed.  The  wood  rays  are  also  shown  extending 


Year's  growih  m  length 


FIG.  45.    External  features  of  a  lilac  twig 
in  winter 

In  the  lilac  the  terminal  bud  dies  early  and 
is  not  usually  present  in  mature  twigs.  The 
spring  growth  is  produced  from  the  last  two 
lateral  buds  formed  at  the  end  of  the  twig  of 
the  season,  as  in  the  figure 


STEMS,  ROOTS,  AND  LEAVES 


85 


as  radial  lines  in  the  wood.  The  annual  rings  arise  from  the 
cambium  layer  and  increase  the  diameter  of  the  stem  annually, 
thus  stretching  and  cracking  the  bark  jacket  of  older  trees. 
The  wood  of  each  annual  ring  is  divided  into  two  zones,  called 
spring  wood  and  summer  wood.  The  spring  wood  is  distin- 
guishable by  being  porous ;  that  is,  it  is  made  up  of  cells  with 
larger  openings,  which  give  greater  porosity  to  the  spring  wood 
than  to  the  summer  wood. 
These  larger  cells,  or  pores, 
are  the  cavities  of  the  large 
water  ducts,  which  are 
usually  more  numerous  in 
spring  wood,  since  it  is 
formed  when  a  large  sup- 
ply of  water  is  needed  for 
the  growth  of  buds,  leaves, 
and  flowers.  The  summer 
wood,  on  the  contrary,  is 
made  up  of  thick-walled 
cells  (which  are  smaller 
than  those  of  the  spring 
wood)  and  has  fewer  water  FlG 
ducts.  Its  chief  functions 
for  the  stem  seem  to  be 
mechanical  and  storing. 
Since  the  last-formed  and 
denser  summer  wood  of 

one  season  abuts  upon  the  first-formed  porous  spring  wood  of 
the  next  season,  the  junction  line  of  the  annual  rings  is  usually 
clearly  marked.  The  width  of  the  annual  ring  for  any  given 
season  is  determined  in  part  by  the  age  of  the  tree  and  in  part 
by  external  conditions.  Periods  of  drought  or  the  loss  of  leaves 
by  frost  or  by  insects  check  the  annual  growth  and  may  even 
result  in  a  double  ring  of  wood  in  one  season  if  growth  is 
resumed  in  the  latter  part  of  the  summer  after  such  a  checking 
process.  Most  trees  also  grow  slowly  in  extreme  youth  or  old 
age,  while  growth  is  most  rapid  in  the  middle  life. 


Gross  structure  of  a  lilac  shoot  two 
years  of  age 

The  limits  of  the  pith,  wood,  bark,  and  epider- 
mis are  shown  in  the  sector  of  the  stem  at  the 
left.  The  principal  tissue  layers  in  each  of  the 
above  subdivisions  are  shown  at  the  right  in 
the  upper  and  lower  sectors 


86 


GENERAL  BOTANY 


In  sections  cut  across  more  mature  stems  of  trees  like  the  oak 
and  the  alder  the  above  layers  will  be  found  to  be  considerably 
changed  in  their  relative  width  and  structure  (Fig.  47).  In  such 
sections  the  outer,  brown  bark  is  much  thicker  and  is  often  cracked 
or  seamed  by  the  great  increase  in  the  diameter  of  the  central 
wood  cylinder.  The  green  bark  has  also  usually  disappeared 
in  mature  stems  and  forms  a  thin  layer  of  crushed  cells  be- 
tween the  outer  corky  bark  and  the  inner  light  bark,  or  phloem. 


Cork  bark 

Skeletal  tissue  (fibers 
and  sderenchyma) 

Phloem 
Cambium 
Summer  wood 
•Spring  wood 

Wood  ray 


FIG.  47.    Transverse  section  of  an  oak  stem  eight  years  old.   Diagrammatic 

The  central  darker  dotted  portion  is  heartwood ;  the  outer  wood  layers,  with  alter- 
nating light  spring  wood  and  dark  summer  wood,  belong  to  the  sap  wood 

The  wood  cylinder  is  also  differentiated  into  an  outer  light  area 
of  sapwood  composed  of  several  of  the  latest-formed  annual  rings 
of  growth,  and  of  a  darker  central  portion,  the  heartwood,  made 
up  of  the  first-formed  wood  in  the  center  of  the  tree.  The  sap- 
wood  contains  living  cells  in  the  form  of  wood  rays  and  wood 
parenchyma  and  is  active  in  the  conduction  of  water  and  food 
and  in  the  storage  of  food  reserves.  The  heartwood  is  dead  and 
serves  a  purely  mechanical  function  in  supporting  the  tree.  Its 
cell  walls  are  impregnated  with  various  substances  which  change 
their  color  and  increase  their  strength. 


STEMS,  KOOTS,  AND  LEAVES 


87. 


The  wood  rays  are  more  numerous  than  in  young  shoots  and 
vary  greatly  in  length  and  width  according  to  their  position  and 
the  time  of  formation.  This  is  due  to  the  fact  that  as  the  wood 
increases  in  circumference  new  rays  are  started  from  the  cam- 
bium at  various  points  year  by  year.  The  new  rays  supply  a 


Transverse 


Transvers 


Annual 

ring  

Radial  section  Oblique -tangential  section 
a  b 


Quarter -sawing 
c 


Straight  grain 
d 


avy  grain 
f 


FIG.  48.   Various  cuts  of  wood  drawn  to  show  the  grain  and  other 
structural  features 

Consult  the  text  for  an  explanation  of  these  figures 

larger  food-storage  system  for  the  tree,  which,  with  the  disap- 
pearance of  the  pith  and  cortex,  comes  to  be  lodged  more  and 
more  in  the  wood  rays  and  wood  parenchyma  of  the  wood 
cylinder.  The  pith  usually  disappears  very  early  and  is  marked 
by  a  dark  spot  in  the  center  of  the  tree. 

The  above  tissues   and  tissue  layers  have  a  very  different 
appearance  when  observed  in  wood  blocks  or  in  thin  sections  cut  in 


88 


GENERAL  BOTANY 


transverse,  radial,  and  tangential  sections.  Fig.  48  illustrates  the 
appearance  of  the  annual  rings  and  of  the  spring  and  summer 
wood  as  they  appear  in  transverse  and  longitudinal  sections 
of  stems  four  and  five  years  of  age.  In  the  transverse  cuts  at 
the  upper  end  of  Fig.  48,  a  and  £>,  the  spring  and  summer  wood 
appear  as  in  the  oak  (Fig.  47),  but  in  the  radial  (a)  and  oblique 
(5)  sections  they  present  a  very  different  appearance.  In  the 
radial  section  the  spring  and  summer  wood  appear  as  narrow 


FIG.  49.    Sections  of  wood  of  bird's-eye  maple  (Acer  saccharum) 

Transverse  section  at  the  left,  radial  section  in  the  center,  and  tangential  (curly  grain) 

at  the  right  in  the  figure.    The  curly  grain  is  due  to  irregularities  in  the  growth  of 

the  annular  rings.   Photograph  furnished  hy  the  United  States  Forest  Service 

and  wide  vertical  lines  or  bands,  while  the  wood  rays  run  across 
the  grain  as  darker  horizontal  bands.  In  the  oblique-tangential 
section  the  rays  have  a  similar  appearance  in  the  upper  semi- 
radial  portion  of  the  section,  but  in  the  lower  tangentially  cut 
portion  they  show  as  vertical  lines  of  varying  height  and  width. 
This  difference  in  appearance  in  the  wood  rays  is  due  to  their 
shape  and  their  radiate  arrangement  in  the  tree  trunk.  The 
"  silver  grain "  in  finished  woods,  notably  in  quarter-sawed 
oak,  is  due  to  the  wide  wood  rays,  which  have  a  shining, 


STEMS,  ROOTS,  AND  LEAVES 


89 


bandlike  appearance  when  viewed  in  radial  sections.  Fig.  48,  <?, 
illustrates  the  method  of  preparing  and  sawing  logs  for  boards 
having  the  quarter-sawed  effect.  The  logs  are  first  stripped  of 
their  bark  and  squared  as  in  the  figure.  The  first  boards  are 
then  cut  in  the  plane  aa,  which  gives  a  radial  cut.  Shorter 
radial  cuts  are  afterward  secured  by  sawing  in  the  planes  bb 
and  cc.  It  is  evident  that  the  first  cuts  through  the  plane 
aa  give  the  widest  and  most  valuable  quarter-sawed  boards. 


II  III 


iw  i. 


mi  it  ifiiiii 
\i\\\  !!S!im 

HlUllV 

i\\\\\mvt 


FIG.  50.    Section  of  red  oak  (Quercus  rubra) 

Transverse  section  of  oak  wood  at  the  right,  showing  wood  rays  and  annual  rings 
composed  of  porous  spring  and  dense  summer  wrood.  Radial  section  in  the  center, 
with  dark  wood  rays  running  across  the  section.  Tangential  section  at  the  left, 
showing  porous  spring  and  dense  summer  wood.  Photograph  furnished  by  the 
United  States  Forest  Service 

The  above  structures,  seen  in  gross  sections  of  woody  stems, 
are  more  evident  in  finished  woods  and  produce  the  chief  orna- 
mental effects  of  wood  furnishings.  This  is  especially  true  of  the 
contrasts  between  the  heartwood  and  sapwood,  fall  and  spring 
wood,  and  between  the  wood  proper  and  the  wood  rays.  Heart- 
wood  and  sapwood  are  easily  distinguishable  from  annual  rings  in 
furniture  or  room  finishings  for  the  reason  that  each  of  the  former 
includes  several  annual  rings  of  growth,  each  of  which  has  its 


90  GENERAL  BOTANY 

layers  of  summer  and  spring  wood.  The  annual  rings  determine 
the  grain  of  wood,  which  is  said  to  be  coarse  if  the  annual 
rings  are  wide,  and  fine  if  the  rings  are  comparatively  narrow. 
Texture,  which  refers  to  the  coarseness  or  fineness  of  the  wood 
elements,  also  enters  into  the  designation  of  grain  as  coarse  or 
fine.  Again,  the  grain  is  straight  if  the  woody  elements  of  the 
annual  ring  run  straight  up  and  down  (Fig.  48,  d),  or  it  is 
wavy  or  curly  if  these  elements  take  an  undulating  course 
(Fig.  48,  e  and/).  In  this  latter  case  the  grain  may  be  desig- 
nated as  curly,  in  curly  birch,  or  as  bird's-eye,  in  bird's-eye  maple 
(Fig.  49).  Silver  grain  of  wood  is  due  to  the  wood  rays,  espe- 
cially when  the  latter  are  cut  radially,  as  is  the  case  in  quarter- 
sawed  oak  (Fig.  50).  The  grain,  whether  due  to  annual  rings 
or  to  wood  rays,  is  also  greatly  modified  in  appearance  in  fin- 
ished woods  by  different  modes  of  cutting  or  sawing  the  original 
log  or  piece  from  which  the  finished  product  is  prepared.  The 
student  should  in  all  cases  supplement  this  brief  text  descrip- 
tion with  critical  observations  of  finished  woods  displayed  in 
the  furniture  and  the  finishings  of  the  laboratory  or  the  home. 

MICROSCOPIC  STRUCTURE 

In  the  above  discussion  of  the  gross  structure  of  woody 
stems  the  main  layers  have  been  outlined  under  the  terms. bark, 
cambium,  wood,  and  pith.  It  remains,  therefore,  to  discuss  these 
layers  somewhat  more  in  detail  as  they  are  seen  in  thin  section 
under  a  compound  microscope.  In  such  sections  it  may  be 
observed  that  each  of  the  above  gross  subdivisions  of  a  stem  is 
composed  of  one  or  more  groups  of  highly  differentiated  cells 
called  tissues.  Each  tissue  is  a  group  of  cells,  similar  in  structure 
and  in  function,  which  has  differentiated  from  the  products  of 
the  meristem  cells  formed  in  the  bud. 

The  bark  (Fig.  52)  is  a  complex  layer  made  up  of  dead  tissue 
elements,  which  serve  for  protection  and  mechanical  support,  and 
of  living,  active  cells,  which  are  concerned  with  the  life  processes 
of  the  stem.  It  is  composed  of  the  following  tissues :  epidermis, 
cork,  cortex,  and  phloem. 


STEMS,  ROOTS,  AND  LEAVES 


91 


The  epidermis,  in  most  plant  stems,  consists  of  a  single  layer 
of  uniform  cells,  which  serve  to  protect  the  young  stem  until 
the  cork  layers  appear.  As  soon  as  the  cork  is  formed  the  epi- 
dermis is  sloughed  off  and  the  cork  functions  in  its  place.  When 
present  the  epidermal  cells  may  be  seen  to  be  of  uniform  size 


FIG.  61.    Microphotographs  of  sections  of  the  stem  and  wood  of  the 
alder  (Alnus  mollis) 

a,  transverse  section  o'f  a  stem  of  Alnus  three  years  old,  showing  spring  and  summer 
wood,  pith,  and  bark  (microphotograph  by  Dr.  E.  C.Jeffrey) ;  6,  transverse  section 
of  the  wood  of  Alnus ;  c,  tangential  section ;  d,  radial  .section.  Note  the  large  light 
water  ducts  in  all  sections  and  the  difference  in  the  appearance  of  the  wood  rays  in 
the  tangential  and  radial  sections 

and  structure,  with  their  cell  walls  greatly  thickened  on  the 
outer  exposed  surface.  This  outer  cell  wall  is  usually  waxy 
and  impervious  to  water. 

*  The  cork  layer  is  composed  of  thin-walled  cells  which  have 
the  properties  of  ordinary  bottle  cork.  In  young  stems  they 
form  a  thin  layer  of  brown  bark,  which  increases  in  thickness 
as  the  stem  matures. 


92 


GENERAL  BOTANY 


Cork 


Cortex 


Parenchyma 
cells 


cortex  is  composed  of  thin-walled  cells  which  serve  a 
storage  function.  These  cells  are  living  during  the  early  life  of 
a  tree,  but  they  are  ultimately  crushed  by  the  growth  in  diameter 

of  the  stem  or  -branch,  and  then 
they  form  a  part  of  the  dead 
tissue  of  the  bark. 

The  phloem  is  early  differen- 
tiated into  two  distinct  layers  in 
most  common  trees  and  shrubs. 
The  outer  layer  is  composed 
of  cells  with  greatly  thickened 
wails,  which  are  termed  phloem 
fibers,  since  in  long  sections  these 
thick-walled  strengthening  cells 
are  seen  to  be  long,  pointed, 
fibrous  cells.  These  fibers  are 
of  special  interest,  since  it  is  from 
such  cells  that  the  commercial 
fibers  of  flax,  hemp,  etc.  are  de- 
rived. Their  great  strength  and 
flexibility  make  them  invaluable 
to  the  plant  as  well  as  to  man. 
We  shall  learn  also  that  they  are 
always  favorably  located  for  the 
greatest  efficiency  in  supporting 
delicate  stems  and  soft  tissues. 
The  inner  phloem  layer  is  com- 
posed of  large  conducting  cells, 
joined  end  to  end,  called  sieve 
tubes,  and  between  them  smaller 
living  cells  called  phloem  paren- 
chyma. The  chain  of  cells  which  together  constitute  each  sieve 
tube  is  so  named  on  account  of  the  fine  perforations  in  the 
transverse  walls  of  the  cells,  composing  the  sieve  plates,  which  are 
supposed  to  facilitate  the  flow  of  soluble  food  materials  up  and 
down  the  stems.  The  small  cells  between  the  sieve  tubes,  being 
also  living  cells,  serve  both  for  the  storage  of  starch  and  for  the 


Phloem 


Cambium 


Xylem 

Wood  fiber 
1Voo~d  parenchyma 

FIG.  52.    Microscopic    structure    of 

the  stem   of  the   alder  (Alnus)  in 

transverse  section 

The  figure  represents  the  camhium  in 
the  fall  and  winter  condition,  after  the 
year's  growth  was  completed.  Note 
the  regular  rows  of  xylem,  phloem, 
and  wood-ray  tissue  cells,  originating 
from  definite  portions  of  the  camhium 
layer 


STEMS,  ROOTS,  AND  LEAVES 


93 


vertical  conduction  of  sugar  through  the  stem.  The  inner  phloem 
as  a  whole  is  therefore  primarily  a  food-conducting  layer,  as  dis- 
tinguished from  the  wood,  which  conducts  water  and  soil  salts. 

The  wood,  or  xylem,  is  much  more  complex  than  the  phloem 
and  is  composed  of  a  great  variety  of  living  and  lifeless  cells 
which  perform  storage,  conducting,  and  mechanical  functions  for 


Wood  ray 


Duct 


Transverse  watt 

Cell  wall 
Cytoplasm 

Sieve  plate 


Sieve  tube 


r  i  uets    ^^-Ducts    Cambium 

Wood  fibers  Wood  Wood  tubes 

parenchyma        parenchyma 


FIG.  53.    Microscopic  structure  of  the  wood  of  alder  in  radial  long  section 

a,  a  drawing  of  a  radial  section  of  the  wood  of  the  alder.  The  living  tissues  have  the 
protoplasm  and  nuclei  dotted.  The  sieve  tuhes  have  the  sieve  plates  on  the  transverse 
walls  separating  adjacent  cells  of  the  tubes,  b,  sectional  views  of  a  duct  and  of  a 
sieve  tube.  Compare  the  sectional  views  of  the  transverse  walls  of  the  duct  and 
sieve  tube  in  6  with  the  surface  views  of  these  same  transverse  walls  «hown  in  a. 

the  plant.  Only  the  more  important  of  these  tissue  cells  need 
be  described  here.  The  water  ducts,  or  trachece,  are  the  most 
conspicuous  elements  of  the  wood;  these,  as  we  have  already 
learned,  are  characteristic  of  the  porous  spring  wood.  In  stems 
like  the  alder  the  water  ducts  appear  like  large  light  holes  in 
thin  sections  of  the  wood  (Fig.  51,  a).  Microscopic  observation 
will  show,  however,  that  they  are  greatly  enlarged  cells  with 
thickened  walls  and  without  living  contents.  In  long  sections 
(Fig.  51,  c  and  d,  and  Fig.  53),  the  ducts  will  be  found  to  be 


94  GENERAL  BOTANY 

composed  of  chains  of  cells  in  which  the  transverse  walls  have 
wholly  or  partly  disappeared,  thus  forming  a  long  tube  for  the 
conducting  of  water.  These  water  ducts  are  usually  not  more  than 
three  or  four  inches  in  length,  although  they  may  be  from  three 
to  six  feet  in  length  in  oaks  and  many  feet  in  length  in  some 
climbing  plants.  The  first-formed  ducts  are  also  characterized 
by  thickenings  on  their  lateral  walls  in  the  form  of  spirals,  rings, 
and  netlike  or  reticulate  thickenings.  In  the  later-formed  ducts 
these  walls  are  often  marked  with  peculiar  pits,  which  give  the 
name  dotted  ducts  to  such  water  vessels.  These  thickenings  on 
the  wall  of  a  duct  serve  to  strengthen  it,  while  the  thin  places 
between  the  thickenings  allow  water  and  salts  to  move  later- 
ally through  the  duct  wall.  The  ducts  are  therefore  admirably 
adapted  for  strength  and  for  the  rapid  conduct  of  water.  Between 
the  ducts  are  the  smaller  fibrous  cells  of  the  xylem,  some  of 
which  are  living,  while  others  are  dead.  The  living  cell  elements 
are  termed  wood  parenchyma ;  they  either  serve  for  storage  of 
reserve  foods  during  periods  of  inactivity  or  they  conduct  foods 
locally  to  and  from  the  wood  rays.  These  wood-parenchyma 
cells  are  usually  thick-walled,  with  fine  perforations  in  the  cell 
walls,  through  which  protoplasmic  strands  connect  the  proto- 
plasts of  adjacent  cells.  The  lifeless  elements  are  fibrous  cells 
resembling  closely  those  of  the  phloem  and  serve  mainly  to 
strengthen  the  stem.  In  some  trees,  however,  they  remain  living 
for  a  considerable  time  and  serve  for  storage,  while  in  others 
they  conduct  water  and  soil  salts.  The  walls  of  wood  fibers  are 
usually  thick,  with  minute  pits  or  pores  which  mark  thinner 
places  in  the  cell  wall. 

The  wood  rays  are  composed  of  thin-walled  living  cells  with 
their  long  axes  running  radially  in  the  stem.  In  a  tangential  sec- 
tion of  wood  the  rays  are  very  numerous  and  are  made  up  of 
more  or  less  elliptical  masses  of  living  cells.  In  radial  view  they 
are  plates  of  cells  running  across  the  section.  Their  primary 
function  is  the  conduction  and  storage  of  organic  food,  although 
they  serve  as  lateral  water  carriers  in  some  plants. 


STEMS,  ROOTS,  AND  LEAVES  95 

THE  CAMBIUM  LAYER  AND  ANNUAL  GROWTH  IN  THICKNESS 

We  have  already  become  familiar  with  the  structure  of  the 
cambium  and  of  the  annual  layers  of  phloem  and  xylem  which 
are  formed  by  it  each  season.  We  may  now  proceed  to  discuss 
more  in  detail  the  formation  and  development  of  the  tissue  ele- 
ments which  comprise  the  phloem  and  xylem. 

The  cambium  is  a  cylindrical  layer  of  actively  dividing  cells, 
which  functions  much  like  the  apical  meristems  of  the  root  and 
the  stem.  The  cells  of  the  cambium  layer  divide  repeatedly 
during  the  spring  and  summer  months,  and  the  products  of  this 
division  are  gradually  transformed  into  the  permanent  sieve 
tubes,  ducts,  and  other  tissue  elements  of  the  annual  layers  of 
the  phloem  and  xylem. 

The  cambium  in  Fig.  52  is  only  a  few  cells  in  width,  owing 
to  the  fact  that  the  section  from  which  the  drawing  was  made  was 
cut  from  a  twig  gathered  late  in  the  autumn.  The  cambium  is 
bounded  on  its  inner  side  by  the  regular  rows  of  cells  comprising 
the  tissue  elements  of  the  xylem,  and  on  the  outside  by  the  sieve 
tubes  and  accompanying  cells  of  the  phloem. 

When  active  growth  begins  in  the  spring,  the  cells  of  this  nar- 
row cambium  zone  increase  greatly  by  cell  division,  so  that  the 
cylinder  becomes  several  cells  in  thickness.  The  outer  layers  of 
new  cells  thus  produced  then  become  gradually  transformed  into 
the  sieve  tubes  and  the  living  and  fibrous  elements  of  the  phloem, 
while  the  inner  layers,  in  a  similar  manner,  become  transformed 
into  the*  ducts  and  other  tissue  elements  of  the  xylem.  Since, 
however,  many  more  layers  of  cambial  cells  become  transformed 
into  xylem  elements  than  into  phloem  during  one  growing  season, 
the  rings  of  xylem  are  much  wider  than  those  of  the  phloem. 
This  process,  repeated  each  year,  finally  produces,  in  old  trees,  a 
thick  cylinder  of  wood  covered  by  a  comparatively  narrow  layer 
of  bark.  The  usual  period  for  the  production  of  wood  and  phloem 
by  the  cambium  seems  to  range,  in  temperate  climates,  from  about 
the  fifteenth  of  April  to  the  fifteenth  of  August  or  the  first  of 
September,  although  phloem  may  be  formed  after  these  dates  in 
some  trees. 


96 


GENERAL  BOTANY 


The  development  of  the  tissue  cells  of  the  phloem  and  xylem 
follows  very  closely  the  stages  of  cell  growth  and  differentiation 
already  traced  in  the  root  tip.  The  cell-division  stage  takes  place 
very  largely  in  the  cambium  layer  (Fig.  54),  corresponding  to  the 
meristem  of  the  root  tip.  This  cell-division  stage  is  followed  by 
a  period  of  growth  in  diameter  and  by  changes  in  cytoplasm  and 


Xylem                        Cambium                       Phloem 

fib 

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due 

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Permanent  tissue      Growing       Cell  division      Growing  ^      Permanent  tissue 
and  maturing  and  maturing 

FIG.  54.   The  development  of  sieve  tubes,  fibers,  ducts,  and  parenchyma  from 
the  cambium.    Diagrammatic 

Each  of  the  above  elements  of  the  phloem  and  xylem  is  represented  in  three  stages, 
namely,  as  a  part  of  the  cambium,  as  a  growing  and  maturing  cell  or  cell  chain,  and 
as  a  permanent  element  of  either  the  xylem  or  the  phloem.  The  cambium  is  regarded 
as  a  cylindrical  meristem  layer,  which,  like  the  apical  meristems,  produces  new 
cells  by  division.  Consult  the  text  discussion  of  this  figure 

cell  walls  which  may  be  designated  as  the  growth  and  maturing 
stages  for  both  the  phloem  and  the  xylem  elements.  The  growth 
and  maturing  stages  are  followed  in  each  tissue  cell  or  cell 
complex  by  the  permanent  tissue  elements. 

We  may  now  consider  in  greater  detail  the  changes  which 
occur  during  the  growth  and  maturing  stages  of  the  living  cell 
elements,  the  fibrous  elements,  and  the  ducts. 

Xylem  elements.  In  the  case  of  the  cells  which  will  ultimately 
form  the  living  storage  parenchyma  cells  very  little  change  takes 
place  during  the  growth  and  maturing  stage,  except  a  slight 


STEMS,  KOOTS,  AND  LEAVES  97 

thickening  of  the  cell  walls  and  some  increase  in  diameter.  The 
cells  destined  to  form  the  strengthening  fibers  of  the  wood 
undergo  much  greater  changes  in  form  and  in  the  thickening  of 
their  cell  walls  to  produce  the  pointed,  thick-walled  permanent 
fibers.  These  fibrous  cells  also  lose  their  living  contents  and  are 
finally  useful  only  as  mechanical  tissue  elements  of  the  wood. 

The  vertical  rows  of  cells  %  produced  by  the  cambium  which 
are  to  form  the  ducts  undergo  very  great  changes  in  size,  in 
cell  contents,  and  in  the  character  of  the  cell  walls.  Unlike  the 
fibrous  cells  the  ducts  are  compound  structures  formed  of  a 
vertical  row,  or  chain,  of  cambium  cells  joined  end  to  end.  Each 
one  of  these  cells  enlarges  greatly  in  diameter  during  the  matur- 
ing stage,  and  the  protoplasm  begins  to  deposit  new  cell-wall 
substance  in  definite  patterns  on  the  thin  cell  wall  of  the  original 
cambium  cell.  These  new  cell-wall  deposits  may  take  the  form 
of  spiral  or  reticulate  (netlike)  thickenings,  or  they  may  take 
place  over  most  of  the  cell  wall,  leaving  thinner  places,  called 
simple  pits,  on  the  cell  wall.  These  simple  pits  then  become 
arched  over,  forming  what  are  called  bordered  pits.  When  the 
growth  in  diameter  and  the  thickening  of  the  walls  of  the  duct 
are  completed,  the  protoplasm  dies  and  the  transverse  walls 
between  the  constituent  cells  partially  or  wholly  disappear. 
These  changes  finally  result  in  a  long,  tubular  duct,  which 
allows  the  easy  vertical  flow  of  water  and  soil  salts.  The  thin 
portions  of  the  lateral  walls  also  allow  of  lateral  migration  of 
fluids,  while  the  thickened  parts  secure  strength  to  the  tree  and 
prevent  the  collapse  of  the  duct  when  the  stem  is  bent  by  the 
wind  or  by  other  agencies. 

Phloem  elements.  The  fibers  and  living  cells  of  the  phloem 
not  directly  connected  with  the  sieve  tubes  have  much  the  same 
history  as  that  just  outlined  for  similar  elements  of  the  xylem. 
The  sieve  tubes,  however,  have  a  somewhat  different  series  of 
developmental  stages.  The  mother  cells  of  sieve  tubes,  which  are 
formed  by  the  cambium,  divide  once  after  their  formation,  giving 
rise  to  a  future  sieve-tube  cell  and  a  living  companion  cell.  The 
companion  cell  undergoes  very  little  farther  differentiation,  but 
the  sieve-tube  cells,  like  the  cells  composing  the  ducts  of  the 


98  GENERAL  BOTANY 

xylem,  increase  greatly  in  diameter  and  undergo  marked 
changes  in  the  character  of  their  cell  walls.  This  latter  change 
consists  in  the  formation  of  the  so-called  sieve  plates  by  perma- 
nent perforations  of  the  transverse  walls  between  a  vertical  row 
of  cells  which  unite  to  form  the  sieve  tube. 

The  sieve  tubes  in  the  permanent  condition  are  therefore  like 
water  ducts  in  being  composed  of  a  vertical  chain  of  greatly 
enlarged  cells  adapted  to  the  rapid  conduction  of  materials  for 
the  use  of  the  plant.  In  the  case  of  the  sieve  tubes  the  materials 
conducted  are  soluble  nitrogenous  foods,  with  probably  some 
sugars  and  other  substances.  The  perforations  in  the  sieve 
plates  allow  this  material  to  move  with  less  obstruction  than 
would  otherwise  be  possible.  The  sieve  tubes,  unlike  the  water 
ducts,  retain  their  cytoplasmic  contents  during  their  active  life 
but  lose  their  nuclei.  They  are  connected  with  the  companion 
cells  by  perforated  areas  in  the  side  walls,  resembling  somewhat 
the  sieve  plates. 

Wood  rays.  The  wood-ray  cells  arise  from  the  cambium,  like 
the  other  tissues  of  the  xylem  and  phloem,  but  undergo  less 
change  in  form  and  structure  than  the  sieve  tubes  and  the 
ducts.  They  are  really  horizontal  cells  of  the  phloem  and  xylem, 
corresponding  in  the  main,  in  structure  and  function,  to  the  living 
cells  already  described. 

The  above  account  illustrates  the  similarity  between  the  grow- 
ing layers,  or  meristems,  in  all  parts  of  the  plant  body  and  the 
developmental  stages  of  cells  which  become  transformed  from  the 
meristem  condition  into  the  permanent  tissues  of  the  plant. 

THE  GENERAL  FUNCTIONS  OF  THE  STEM  TISSUES 

Storage.  If  transverse  sections  of  living  twigs  or  stems  are 
cut  in  late  autumn  or  winter  and  stained  with  iodine  solution, 
the  abundant  storage  of  starch  can  easily  be  demonstrated. 
This  starch  storage  occurs  mainly  in  the  thin-walled  cells  -of 
the  cortex,  phloem,  xylem,  and  pith.  In  the  xylem,  or  wood, 
the  starch  appears  abundantly  in  the  wood-ray  cells  and  in  the 
wood  parenchyma.  These  cells  often  form  continuous  bands  of 


STEMS,  KOOTS,  AND  LEAVES  99 

tissue  connected  with  the  wood  rays  and  with  the  ducts.  This 
distribution  of  the  starch-storage  cells  in  the  wood  enables  the 
living  cells  to  transport  the  sugar,  which  forms  from  the  starch 
in  the  spring,  along  almost  continuous  paths  to  the  cambium 
layer  and  the  phloem.  The  other  reserve  foods,  such  as  fats  and 
nitrogenous  substances,  are  less  easily  demonstrated  in  wood, 
but  such  reserves  are  laid  up  with  the  starch  in  trees  in  the  thin- 
walled  tissues  mentioned  above.  Toward  spring,  when  the 
growth  of  the  tree  begins,  the  solid  reserves  are  converted  into 
soluble  sugars,  fats,  and  proteins,  and  migrate  by  osmosis  to  the 
cambium,  growing  buds,  and  root  tips,  where  they  are  converted 
into  protoplasm  and  cell  walls. 

Conduction.  The  soluble  foods  just  mentioned  move  horizon- 
tally along  the  wood  rays  and  upward  or  downward  in  the 
wood  parenchyma  and  the  phloem,  as  the  case  may  be.  The 
nitrogenous  substances  appear  to  move  mainly  in  the  sieve  tubes, 
while  the  soluble  sugars  migrate  in  the  phloem  parenchyma. 
The  method  of  movement  is  by  osmosis,  as  already  indicated, 
and  the  direction  may  be  upward  toward  the  terminal  buds, 
downward  toward  the  root  tips,  or  outward  into  horizontally 
placed  branches  and  lateral  buds.  Mass  movement  also  occurs 
in  sieve  tubes  when  the  bending  of  a  tree  by  the  wind  squeezes 
the  sieve  tubes  and  forces  the  food  in  them  to  move  along. 
Sugar  also  moves  in  the  ducts  in  certain  trees  like  the  maple 
and  birch.  In  a  tree  or  other  plant  there  is  not,  therefore,  any 
definite  circulation  of  foods,  but  rather  a  general  movement 
from  places  of  storage  to  places  of  growth,  or  from  places  of 
manufacture  to  storage  tissues.  The  water  and  soil  salts  move 
upward,  in  a  stem,  from  the  roots  to  the  leaves.  The  main  chan- 
nels are  the  great  water  ducts,  although  in  many  trees  some  of  the 
fibrous  cells,  called  tracheids,  serve  as  conductors  with  the  ducts. 
In  trees  of  the  pine  family  these  shorter  wood  cells  (tracheids) 
are  almost  the  sole  conductors  of  water.  The  great  water 
and  food  streams  in  a  tree  have  therefore  quite  different  paths, 
so  that  they  do  not  interfere  with  each  other.  In  summer,  when 
the  foods  are  being  constructed  in  the  leaves,  the  soluble  foods 
move  downward  in  the  phloem  to  be  stored  in  the  wood  rays, 


100  GENERAL  BOTANY 

pith,  phloem,  and  cortex.  The  main  water  stream  at  the  same 
time  moves  upward  to  supply  the  leaves  with  water  and  prevents 
them  from  drying  up  by  the  constant  loss  of  water  vapor. 

In  early  spring  and  summer  the  food  stream  moves  in  reverse 
directions  to  supply  the  growth  needs  of  the  tissues,  while  the 
water  stream  again  moves  upward  and .  supplies  water  for  the 
inflation  of  the  growing  cells  in  buds  and  in  the  cambium  layer. 

Supporting  and  protective  function.  We  have  already  learned 
that  the  early  formation  of  cork  and  bark  in  the  outer  layers  of 
twigs  and  shoots  soon  replaces  the  epidermis  as  a  protective 
external  layer.  The  continued  formation  of  cork  and  of  the 
supporting  layers  of  the  phloem  finally  produce  in  old  trees  a 
highly  protective  bark  which  shields  the  delicate  tissues  within 
the  tree  from  mechanical  injury  and  guards  against  excessive 
loss  of  water  and  sudden  changes  in  temperature. 

As  we  have  already  seen,  the  supporting,  or  mechanical,  func- 
tion is  provided  for  by  thick-walled  cells  or  fibers,  both  in  the 
phloem  and  in  the  wood.  In  older  trees  and  shrubs  the  thick- 
walled  cells  of  the  wood  cylinder  are  the  real  supporting  tissues, 
while  the  mechanical  layers  of  the  phloem  serve  to  strengthen 
the  outer  bark  jacket. 

GENERAL  STRUCTURE  AND  PHYSIOLOGY  OF  TREES 

SUMMARY 

Structural  features  of  the  tree  in  longitudinal  section.  The  main 
facts  noted  in  the  previous  pages  can  now  be  summarized  and  cor- 
related by  using  diagrammatic  drawings  (Fig.  55,  A  and  5),  as 
illustrations  of  the  main  structural  features  involved  in  the  growth 
and  development  of  a  tree.  In  Fig.  55,  A,  the  tissues  of  the  main  trunk 
and  branches  of  a  tree  are  so  drawn  as  to  display  in  their  proper 
relations  the  annual  rings  of  growth,  the  wood  rays  in  radial  view, 
and  the  differentiation  of  the  woody  cylinder  into  a  central  area  of 
heartwood  and  an  outer  lighter  zone  of  sapwood.  In  addition  to 
these  structures  the  figure  shows  the  clos.e  connection  of  the  wood 
and  pith  of  the  lateral  branches  with  similar  structures  of  the  main 
axis.  The  student  will  also  observe  that  each  so-called  annual  ring 
of  wood  is  really  a  cone-shaped  cylinder  laid  down  by  the  cambium 


Epidermis 
'  Cortex 
Phloem 
'Xylan 
a    'Pith 


rk  bark 
Cortex 
^Phloem 
-Xylem 
-Pith 

Wood  ray 
•ambium 


Bark 
Wood  ray 

-Annual  ring 
-Pith 

Pruned 
branch 


Covered^ 
branch 


FIG.  55.    The  structure  and  growth  of  a  tree 

A,  a  diagram  illustrating  the  gross  anatomy  of  an  oak  tree  in  long  section.  Note  the 
relation  of  the  annual  rings,  the  junction  of  the  branches  with  the  main  trunk,  and 
the  covering  of  pruned  branches.  J3,  transverse  sections  of  the  tree  shown  in  A,  at 
three  levels.  These  latter  figures  are  designed  to  illustrate  the  early,  or  primary, 
structure  (a)  and  the  secondary  changes  (&,  c)  due  to  the  production  of  wood  and 
bark  by  the  cork  and  the  fascicular  cambiums.  Consult  the  text  for  a  discussion  of 

these  changes 


.10.2    c  i  C  GENERAL  BOTANY 

each  season  upon  the  outer  face  of  the  wood  of  the  previous  year, 
In  the  season  in  which  any  given  wood  cylinder  was  formed  it  ex- 
tended from  the  base  of  the  tree  to  the  apical  meristem  at  its  apex, 
where  the  new  wood  cylinder  terminated  in  the  meristem  of  the 
apical  bud  of  the  season.  A  branch  has  been  covered  by  the  forma- 
tion of  new  tissue  over  it,  and  lateral  buds  are  shown  on  the  exterior 
of  the  tree  above.  The  drawing  thus  presents  a  real  picture  of  the 
main  structural  features  of  a  tree  as  they  are  revealed  in  a  median 
longitudinal  section  through  the  main  axis  and  its  branches. 

Growth  in  diameter  and  the  formation  of  bark.  The  gradual  growth 
of  the  tree  in  diameter  through  the  agency  of  the  cambium  is 
illustrated  in  the  three  sections  of  the  tree  cut  at  different  levels 
of  the  main  trunk  (Fig.  55,  J5).  Fig.  55,  B,  a,  is  a  section  through  the 
young  terminal  twig  as  it  is  developing  in  the  spring  from  a  ter- 
minal bud,  and  represents  what  is  often  called  the  primary,  or  early, 
structure  of  the  shoot.  The  epidermis  is  here  intact  and  forms  the 
only  protective  layer  of  the  developing  shoot.  The  cortex  and  pith 
are  wide  and  are  composed  of  thin-walled  living  storage  parenchyma. 
The  vascular  cylinder  of  phloem  and  xylem  is  narrow,  and  in  many 
instances  forms  a  broken  ring  of  vascular  bundles,  as  seen  in  a 
transverse  section,  with  bands  of  parenchyma  tissue  joining  the 
pith  and  cortex. 

At  a  lower  level  of  the  tree  trunk  very  material  changes  have 
taken  place  in  the  above  tissue  layers,  owing  to  the  formation  of  an 
outer  corky  bark  and  to  the  great  increase  in  diameter  of  the  vascu- 
lar ring  of  phloem  and  xylem.  The  outer  layer  of  corky  bark  was 
formed  by  a  cork  cambium  developed  by  the  cell  layer  immediately 
beneath  the  epidermis.  This  cell  layer  begins  to  divide  during  the 
first  season  in  the  growth  of  most  young  shoots  and  forms  a  cylin- 
drical layer  of  cambial  cells  beneath  the  epidermis,  which  then  be- 
comes transformed  into  dead,  brown  cork  cells.  It  is  for  this  reason 
that  shoots  change  their  color  from  green  to  brown  during  the  first 
season  of  their  growth.  As  soon  as  the  cork  is  formed  the  epidermis 
dies  for  lack  of  water  and  is  finally  sloughed  off.  A  shoot  in 
the  condition  shown  in  Fig.  55,  B,  b,  would  present  the  external  brown 
bark,  the  green  bark,  or  cortex,  and  the  inner  light  bark,  or  phloem, 
outside  of  the  wood  ring,  as  shown  in  the  section  of  the  lilac  (Fig.  46). 

Structure  of  the  tree  at  maturity.  Fig.  55,  Z?,  c,  shows  the  structural 
features  of  the  old  stem  well  down  toward  the  base  of  the  tree. 
The  outer  brown  bark  has  here  been  replaced  by  a  thick  bark  jacket 


STEMS,  KOOTS,  AND  LEAVES  103 

composed  of  three  layers.  The  outer  layer  is  the  greatly  thickened 
cork  layer,  which  is  now  seamed  and  cracked  by  the  pressure  exerted 
by  the  expanding  wood  cylinder.  The  middle  layer  is  the  remnant 
of  the  former  green  bark,  or  cortex,  now  composed  of  dead  tissue 
squeezed  between  the  outer  cork  layer  and  the  phloem.  The  outer 
portions  of  the  phloem  form  the  inner  layer  of  the  dead  bark,  made 
up  of  layers  of  thick-walled  phloem  fibers  alternating  with  crushed 
and  dead  sieve  tubes  and  phloem  parenchyma.  This  outer  portion 
of  the  phloem  constitutes  a  kind  of  exoskeleton  for  the  tree, 
forming  with  the  other  layers  of  the  bark  an  effective  protection 
against  the  loss  of  water  and  heat,  as  well  as  a  means  of  warding 
off  mechanical  injury.  The  inner  phloem  is  replaced  each  year  by  a 
new  layer  produced  by  the  cambium.  This  new  annual  layer  is  the 
effective  portion  of  the  phloem  in  conducting  and  storing  food,  since 
the  average  life  of  the  sieve  tubes  in  most  trees  is  limited  to  a  single 
season.  The  central  portion  of  the  wood  cylinder  has  been  converted 
into  dead  heartwood,  and  the  pith  has  disappeared.  The  sapwood 
now  forms  the  living,  active  portion  of  the  wood,  furnished  with  the 
three  main  systems  of  tissues  discussed  in  the  preceding  pages : 
namely,  the  conducting  water  ducts ;  the  storage  system  of  living 
cells,  made  up  of  wood  rays  and  xylem  parenchyma ;  and  the  mechan- 
ical fibrous  elements  constituting  the  endoskeleton. 

Longevity  of  the  tree.  The  tree,  unlike  any  othef^known  organ- 
ism, is  so  constituted  as  to  be  able  to  perpetuate  its  life  through  an 
almost  indefinite  period  of  time.  A  new  crop  of  leaves  is  formed  each 
season  for  the  making  of  food  by  photosynthesis,  while  the  roots 
invade  a  constantly  increasing  area  of  soil  for  the  absorption  of  water 
and  solutes  in  the  form  of  soil  salts.  The  supporting  trunk  is  enabled, 
through  its  outer  and  inner  cambium  layers,  to  increase  the  thickness 
and  effectiveness  of  its  protective  bark  and  also  of  its  inner  skeleton  of 
xylem  and  phloem  fibers.  At  the  same  time,  as  the  central  wood  be- 
comes transformed  into  dead  mechanical  elements,  the  cambium  forms 
new  layers  of  conducting  and  storing  tissues  in  the  sapwood,  which 
keep  pace  with  the  increase  of  its  absorbing  and  manufacturing  sur- 
faces. This  power  of  constant  growth,  throughout  a  period  extending 
over  hundreds  of  years  in  the  case  of  the  longest-lived  trees,  is 
wholly  due  to  the  fact  that  certain  cell  groups,  which  we  have  called 
meristems  and  cambiums,  retain  a  condition  of  perpetual  youth  and 
activity,  which  enables  them  to  supplement  dead  and  useless  tis- 
sues with  an  annual  growth  of  new  and  active  ones.  The  long  period 


104  GENERAL  BOTANY 

necessary  for  the  growth  of  a  young  tree  from  the  seed  is  thus 
compensated  for  by  its  extraordinary  life  period  when  it  is  once 
established. 

Physiology  of  the  tree.  During  the  summer  months  water  is  being 
absorbed  by  the  roots  and  passed  up  the  great  water  ducts  to  the 
living  tissues  of  the  tree  trunk  and  to  the  leaves,  where  a  small 
part  is  used  in  the  manufacture  of  food  and  a  very  large  part  is 
evaporated.  During  the  entire  summer  season  the  leaf  is  making 
food  in  the  form  of  sugar,  starch,  and  soluble  nitrogenous  food  sub- 
stances. A  part  of  this  food  is  used  at  once  by  the  living  tissues  of 
the  leaf,  but  most  of  it  is  transported  back  through  the  phloem  into 
the  stem  and  stored  in  the  wood  rays  and  the  wood  parenchyma. 
The  food  moves  down  the  veins  and  vascular  tissue  of  the  leaf 
petiole  into  the  phloem  of  the  branches  and  main  tree  trunk.  In  the 
phloem  the  sieve  tubes  serve  to  convey  the  greater  part  of  the  nitrog- 
enous food  materials,  while  the  sugars  move  in  the  phloem  paren- 
chyma and  companion  cells  of  the  sieve  tubes.  When  the  food 
stream  reaches  the  wood  rays  in  the  phloem,  a  portion  of  it  passes 
horizontally  along  the  rays  to  the  xylern,  where  it  is  usually  trans- 
formed into  starch  and  stored  in  the  rays  themselves  and  in  the 
living  wood-parenchyma  cells  of  the  sapwood.  If  the  tree  is  young 
and  still  retains  a  living  pith  and  cortex,  a  portion  of  the  food  may 
pass  into  these  tissues  and  be  stored.  Trees  in  which  the  stored 
food  is  mainly  in  the  form  of  starch  are  often  called  starchy  trees, 
to  distinguish  them  from  trees  like  the  linden,  or  basswood,  in  which 
the  reserved  food  is  mainly  composed  of  fats.  In  either  case  the 
reserve  food  is  converted  into  sugar,  by  digestion,  before  it  is  circu- 
lated and  used  for  growth  and  repair.  When  spring  growth  begins, 
before  the  advent  of  the  leaves,  the  food  currents  are  reversed  and 
pass  outward  from  the  rays  and  wood  parenchyma  to  supply  the 
cambium  for  its  growth.  In  a  similar  manner  food  streams  move 
upward  and  downward  in  the  phloem  to  supply  the  growing  buds 
and  root  tips  with  food  at  the  extremities  of  the  tree.  We  see,  there- 
fore, that  the  tree,  while  its  tissues  provide  adequately  for  the  trans- 
portation of  water  and  foods  and  for  food  storage,  has  no  circulation 
such  as  that  provided  by  the  heart,  arteries,  and  veins  of  the  higher 
animals.  Foods  move  up  or  down,  outward  or  inward,  according  to 
the  needs  of  the  tissues  in  any  given  part.  Water,  on  the  contrary, 
moves  mainly  upward,  although  some  lateral  movement  is  necessary 
in  order  to  supply  the  living  cells  of  wood,  phloem,  pith,  and  cortex. 


STEMS,  ROOTS,  AND  LEAVES  105 

HERBACEOUS  STEMS 
DICOTYLEDONS 

In  the  following  account  of  the  structure  of  herbaceous  dicoty- 
ledons the  stem  of  a  common  cultivated  sage  {Salvia)  will  be 
taken  to  illustrate  the  usual  structure  of  one  type  of  herba- 
ceous stems.  In  studying  sections  through  portions  of  young 
stems  of  Salvia  (Fig.  56,  a)  and  through  the  older  basal  portions 
(Fig.  56,  5)  the  student  will  be  impressed  with  certain  broad 
distinctions  between  such  a  type  of  herbaceous  stem  and  the 
woody  stems  already  considered.  These  general  distinctions  are 
necessarily  concerned  with  the  main  tissue  areas  common  to  all 
stems ;  namely,  the  bark,  cortex,  and  pith,  the  cambium,  and 
the  vascular  cylinder. 

The  bark  is  evidently  quite  lacking  in  Salvia,  if  by  this  term 
we  refer  only  to  the  thick  jacket  of  bark  which  clothes  the  out- 
side of  woody-stemmed  plants.  The  epidermis  is  therefore  per- 
sistent throughout  the  season  and  serves  its  usual  functions  of 
checking  the  loss  of  water  from  the  delicate  tissues  beneath  it 
and  of  protecting  the  stem  from  insects  and  fungi. 

The  cortex  of  the  Salvia  stem  is  a  wide  layer,  as  in  most 
strictly  herbaceous  stems,  and  in  the  younger  portions  its  cells 
contain  green  chloroplasts  which  function  in  photosynthesis.  The 
cortex  cells  are  also  differentiated  into  a  narrow  outer  thick- 
walled  strengthening  cylinder  and  a  wider  inner  cylinder  made 
up  of  more  delicate  cells,  which  perform  the  work  of  photosyn- 
thesis and  storage.  The  pith  is  relatively  larger  than  in  woody 
stems  and  serves  the  usual  storage  function. 

The  vascular  cylinder  is  perhaps  the  most  distinctive  feature 
of  the  stem  of  Salvia  as  compared  with  the  thick  vascular  cylinder 
of  trees  and  shrubs.  In  sections  of  young  stems  (Fig.  56,  a)  the 
vascular  tissue,  composed  of  phloem  and  xylem,  is  seen  to  occur 
in  isolated  strands,  called  vascular  bundles,  united  by  thin  bands 
of  fiberlike  cells,  easily  distinguished  from  the  adjacent  cells  of 
the  pith  and  cortex  by  their  smaller  size  in  transverse  section 
and  by  their  considerable  length  as  seen  in  long  sections.  These 
connecting  bands  or  plates  of  fibrous  tissue,  together  with  the 


106 


GENERAL  BOTANY 


vascular  bundles,  form  a  complete  cylinder  separating  the  pith 
from  the  cortex    and  comparable  to    the   thicker  phloem   and 

xylem   cylinder    of 
•Skeletal  tissue 
^-Epidermis 
t-Cortex 
•^Cambium 
'Xylem 

"-Pith 


woody  plants. 

The  structure  of 
the  young  stem 
of  Salvia  outlined 
above  is  very  simi- 
lar to  that  observed 


Skeletal 
tissue 
Epidermis 
Cortex 
Phloem 
Cambium 


-Pith 


in  young,  growing 
stems  of  herbaceous 
and  woody  plants. 
In  some  herbaceous 
plants  the  vascu- 
lar bundles  remain 
completely  isolated 
throughout  the  life 
of  the  plant,  even 
in  older  portions 
of  the  stem,  and 
within  these  bun- 
dles very  little  sec- 
ondary growth  of 
xylem  and  phloem 
takes  place  from 
the  cambium.  In 
older  stems  of  the 
Salvia  (Fig.  56,  5)  a 
solid  vascular  ring 
of  xylem,  phloem, 
and  cambium  is  to 
be  seen,  resembling 
closely  the  thicker 
vascular  cylinder  of  woody  plants.  This  complete  cylinder  of 
vascular  tissue  in  the  older  stems  of  Salvia  is  produced  by  the 
cambium,  which,  as  the  stem  matures,  forms  across  the  fibrous 
tissue  bands  separating  the  primary  bundles,  and  starts  the 


FIG.  56.    Structure   of  the  young  and  mature  stem 
of  a  common  sage  (Salma) 

a,  a  transverse  section  of  a  young  stem  of  Salvia.  The  sec- 
tion illustrates  the  structure  of  herbaceous  stems  as  com- 
pared with  wood  stems,  like  that  of  the  alder  and  oak ; 
6,  a  transverse  section  of  an  older  stem  of  Salvia.  Secon- 
dary thickening  by  means  of  the  cambium  has  taken 
place,  forming  a  wider  zone  of  phloem  and  xylem 


STEMS,  ROOTS,  AND  LEAVES  107 

production  of  new  secondary  phloem  and  xylem  in  these  regions. 
This  new  phloem  and  xylem,  which  is  common  in  the  older 
portions  of  most  herbaceous  stems,  is  composed  of  the  same 
kinds  of  tissue  elements  as  those  produced  by  the  cambium 
in  woody  stems.  Some  of  the  tissue  elements  —  for  example, 
ducts  and  wood  rays  —  have  certain  features  characteristic  of 
herbaceous  stems,  but  these  differences  are  not  significant  for 
our  general  treatment. 

It  is  also  evident  that  no  annual  rings  of  wood  can  be  pro- 
duced in  an  annual  herbaceous  stem  like  Salvia.  The  above  dis- 
tinctions between  Salvia  as  a  type  of  herbaceous  stem  and  the 
woody  stems  of  trees  previously  studied  may  be  taken  as  gen- 
erally applicable  to  annual  herbaceous  plants  and  perennial  tree 
types.  It  should  be  remembered,  however,  that  between  the 
two  extremes  all  gradations  in  stem  structure  occur,  so  that  it  is 
impossible  to  establish  distinctive  types  of  herbaceous  and  woody 
stem  structure. 

We  have  indicated  above  that  in  some  herbaceous  plants  the 
vascular  strands  remain  isolated  during  the  entire  seasonal  life  of 
the  plant.  In  Salvia  and  its  relatives  there  always  exists  a  com- 
plete cylinder  of  tissue,  including  the  vascular- ^strands,  which 
corresponds  to  the  thick  vascular  cylinder  of  trees.  In  larger 
herbaceous  plants,  like  the  castor  bean  and  the  sunflower,  a  thick 
vascular  ring  is  developed,  approaching  the  condition  found  in 
shrubs  like  the  roses,  currants,  and  barberry.  These  latter  plants 
again  present  a  graduated  series  in  stem  structure  which  bridges 
the  gap  between  the  larger  herbaceous  types  and  the  strictly 
woody  types  of  forest  trees. 

The  stem,  like  other  parts  of  the  plant  body,  is  thus  seen  to 
illustrate  the  principle  of  evolution  in  a  complete  series  of  closely 
related  forms.  Some  botanists  now  maintain  that  the  course  of 
evolution  in  stems  has  been  downward  from  the  trees  of  early 
geologic  times  to  the  herbs  of  the  present  day.  If  this  is  true, 
then  the  vascular  cylinder  of  Salvia  is  simply  a  greatly  reduced 
vascular  cylinder  of  earlier  tree  forms,  in  which  the  mature  stem 
of  the  herbaceous  type  approximates  in  structure  to  young  twigs 
of  a  tree  before  the  outer  corky  bark  and  the  thick  vascular 


108  GENERAL  BOTANY 

cylinder  have  had  time  to  develop.  In  other  words,  the  herba- 
ceous stem  remains  permanently  in  the  twig  stage  of  its  woody 
progenitors. 

With  this  brief  discussion  of  the  intermediate  types  of  stem 
structure,  we  may  return  to  a  summary  of  the  differences  between 
strictly  herbaceous  stems  like  that  of  Salvia  and  the  pure  woody 
types  represented  in  our  common  trees. 

SUMMARY 

1.  The  corky  bark  of  trees  is  absent  in  Salvia,  and  the  epidermis 
is  continuous  throughout  the  seasonal  life  of  the  plant. 

2.  The  cortex  is  wide  and  the  pith  is  correspondingly  large  in  the 
herbaceous  type  in  order  that  they  may  serve  as  storage  areas.    In 
woody  steins  storage  is  more  largely  in  the  wood  rays  and  paren- 
chyma of  the  vascular  cylinder,  since  the  pith  and  cortex  die  early 
in  the  life  of  such  perennial  plants. 

3.  The  vascular  cylinder  is  thin  in  herbaceous  plants,  particularly 
in  the  upper  and  younger  parts  of  the  stem. 

4.  A  cambium  layer  is  present  in  most  herbaceous  stems,  which 
(as  in  Salvia)  produces  a  complete  vascular  cylinder  of  xylem  and 
phloem  in  the  older  portions  of  the  stem. 

5.  Annual  rings  of  growth  are  absent  in  the  aerial  stems  of  these 
plants,  since  their  life  is  limited  to  a  single  season. 

6.  Wood  rays  are  present  in  the  xylem  and  phloem  produced  by 
the  cambium,  but  in  many  herbaceous  stems  they  differ  in  structure 
from  those  found  in  woody  plants. 

MONOCOTYLEDONS 

The  monocotyledonous  stems,  including  corn,  the  cereals,  the 
grasses,  and  the  palms,  have  a  very  different  type  of  stem  from 
that  of  the  dicotyledons  described  above. 

The  distinctive  features  of  the  mature  stem  of  monocotyledons 
may  be  summarized  as  follows : 

The  bundles  (fibro vascular  bundles)  mentioned  in  the  herbaceous 
dicotyledon  as  forming  parts  of  a  dissected  phloem  and  xylem  ring 
are  scattered  throughout  the  stem  in  typical  adult  stems  of  mono- 
cotyledons (Fig.  57,  A).  The  bundles  themselves  have  also  lost  the 


STEMS,  ROOTS,  AND  LEAVES 


109 


cambium  layer,  so  that 
such  stems  have  no 
power  of  increasing  the 
diameter  of  the  stem 
by  cambial  growth. 
There  is  no  distinctive 
pith,  since  the  bundles 
from  the  leaves  arch 
into  the  center  of  the 
stem  and  occupy  por- 
tions of  the  pith  region, 
as  indicated  in  the  fig- 
ure. In  many  mono- 
cotyledons the  cortex 
is  also  difficult  to  dis- 
tinguish, since  the  vas- 
cular bundles  occur  in 
it  and  no  sharp  division 
line  exists  between  cor- 
tex and  pith.  In  these 
unusual  stem  types  the 
storage,  mechanical,  and 
conducting  functions 
are  provided  for  in 
much  the  same  general 
way  as  in  dicotyledons, 
except  that  the  tis- 
sues are  differently  dis- 
posed in  the  stem  and 
in  the  bundles.  The 
structure  of  the  stem 
in  the  seedling  of  a 
monocotyledon  often 
resembles  that  of  the 
dicotyledons,  so  that 
botanists  are  agreed 
that  the  monocotyle- 
don ous  type  of  stem 
originated  from  the 
dicotyledonous  type. 


B 
FIG.  57.    Structure  of  the  stem  of  corn  (Zea  mays) 

A,  gross  structure  of  the  corn  stem:  c,  epidermis; 
fv,  vascular  bundles ;  p,pith;  B,  microscopic  structure 
of  a  vascular  bundle :  /,  parenchyma ;  pr,  early  phloem 
(protophloem) ;  s,  sieve  tubes ;  com,  companion  cells ; 
v,  large  water  ducts;  sp,  spiral  duct;  a,  annular  duct; 
i,  intercellular  passage 


110 


GENERAL  BOTANY 


oot  hair 

.Endodermis 
Phloem 
-Xylem 
Cortex 
Epidermis 


ROOTS 

Structure.  Roots  resemble  stems  in  their  general  structure, 
so  that  little  need  be  repeated  here  concerning  the  cell  elements 
of  the  mam  tissue  areas  of  roots.  The  essential  differences  in 
the  arrangement  of  the  tissues  in  roots  and  stems  is  indicated  in 
the  following  summary  and  figure  relating  to  root  structure. 

The  epidermis  of  roots  (Fig.  58)  is  often  termed  the  piliferous 
layer,  for  the  reason  that  its  cells  may  elongate  in  the  younger 

portion  of  the  root  to 
form  so-called  root  hairs, 
which  are  the  most  im- 
portant absorbing  area 
of  roots.  It  is  estimated 
that  as  many  as  three 
hundred  hairs  may  grow 
on  one  square  millimeter 
of  root  surface,  and  that 
the  absorptive  surface  of 
this  portion  of  the  root 
is  thereby  increased  from 
five  to  ten  times  that  of 
an  equal  naked  root  sur- 
face, or  even  more  than 
that.  These  hairs  cover 
a  limited  surface  of  the 
root  back  of  the  elongating  zorje.  As  the  root  grows  older  the 
hairs  disappear  and  new  hairs  are  formed  nearer  the  tip. 

The  cortex  of  a  typical  root  resembles  that  already  described 
for  the  herbaceous  stem,  except  that  there  is  no  strengthening 
layer  in  roots.  The  inner  layer  of  the  cortex,  called  the  endo- 
dermis, is,  however,  much  better  developed  than  it  is  in  most 
stems.  This  endodermis  in  roots,  like  the  outer  epidermis  of 
stems,  is  practically  impervious  to  water  (except  in  certain  places 
where  the  cells  are  not  of  the  usual  impervious  type),  thus  keep- 
ing the  water  and  food  streams  of  the  phloem  and  xylem  confined 
and  hindering  them  from  flowing  laterally  into  the  cortex. 


FIG.  58.    A  transverse  section  of  a  root 

Note  the  lack  of  pith,  the  central  xylem,  and  the 

radially  arranged  phloem  masses,  characteristic 

of  roots 


STEMS,  ROOTS,  AND  LEAVES 


111 


The  pericyde,  which  is  the  layer  of  cells  just  beneath  the 
endodermis,  is  very  important  in  roots,  since  it  is  the  place  of 
origin  of  cork  and  of  lateral  roots.  When  older  roots  begin  to 
form  cork,  it  does  not  arise  from  the  epidermis  or  from  the  cor- 
tex, as  in  stems,  but  from  the  pericycle.  The  cells  of  the  peri- 
cycle  therefore  act  as  a  cork  cambium  and  thus  originate  cork 
layers  within  the  root  by  cell 
division  and  cell  differentia- 
tion into  cork.  When  one  or 
more  cork  layers  are  formed, 
the  cortex  and  the  epidermis 
dry  up  and  peel  off,  leaving 
the  cork  as  the  outer  brown 
bark  of  the  root. 

Lateral  roots  also  arise 
from  the  pericycle  and  bore 
their  way  through  the  cortex 
and  epidermis.  They  thus 
differ  in  their  origin  from 
the  leaves  and  branches, 
which  arise  by  the  division 
and  differentiation  of  cells 
in  the  epidermis  and  outer 
cortex. 

In  young  roots  the  xylem 
and  phloem  have  the  ar- 
rangement which  is  repre- 
sented in  the  figure.  The 
xylem  occupies  the  center  of  the  root,  thus  excluding  a  definite 
central  pith.  The  phloem  alternates  with  the  radiating  arms  of 
the  xylem  and  so  gives  rise  to  what  is  often  termed  the  radial 
arrangement  of  phloem  and  xylem. 

The  cambium  arises  outside  of  the  xylem  and  inside  of  the 
phloem  masses.  Its  general  method  of  developing  tissues  by  cell 
division  is  identical  with  that  of  the  cambium  of  stems,  already 
described.  The  ultimate  structure  of  old  roots  is  therefore  very 
similar  to  that  of  stems,  except  that  the  center  of  a  mature  root 


FIG.  59.   Root  system  of  a  plant  of  red 
clover  ( Trifolium  pratense) 

Note  the  tendency  of  the  main  roots  to  pene- 
trate to  great  depths  in  the  soil 


112 


GENERAL  BOTANY 


is  not  occupied  by  pith  or  a  pith  cavity.  This  is  due  to  the  fact, 
already  mentioned,  that  the  xylem  of  a  young  root  occupies  the 
center  of  the  root,  excluding  the  pith. 

Functions..  Roots  serve  various  functions,  such  as  anchorage 
in  the  soil  for  the  aerial  portions  of  the  plant  body,  the  transpor- 
tation of  food  and  water,  storage,  and  absorption.  Of  these 
various  functions  the  most  important  for  our  consideration  is 
that  of  absorption,  since  the  other  functions  are  more  or  less 
apparent  after  our  discussion  of  similar  functions  in  stems. 

We  have  already  learned 
that  the  root  system  is 
able  to  adjust  itself  by 
tropistic  movements  so 
as  to  place  its  members 
in  favorable  relations  to 
the  soil  particles  and  soil 
water.  This  adjustment, 
together  with  the  great 
extent  of  the  root  sys- 
tem, enables  both  herba- 
ceous and  woody  plants 
to  absorb  the  large  quan- 
tities of  water  necessary 
to  meet  the  great  demands  made  upon  them  by  the  evaporation 
of  water  vapor  from  the  leaves.  Some  plants,  like  the  clovers, 
have  deeply  penetrating  roots  (Fig.  59),  while  others  (for 
example,  many  of  the  cereal  grains  and  some  trees)  have  sur- 
face roots.  In  alfalfa  the  root  system  is  said  to  penetrate  to 
a  depth  of  31  feet,  and  in  mesquite,  a  desert  plant,  to  that 
of  60  feet.  Corn  (Fig.  60)  has  been  estimated  to  have  a  root 
system  which  reaches  a  total  length  of  over  1300  feet,  and  the 
roots  of  a  single  plant  of  oats  may  be  as  much  as  150  feet  in 
length.  In  many  trees  the  root  system  quite  equals  the  branch 
system  in  horizontal  extension,  and  its  absorbing  root  tips  are 
placed  where  they  receive  the  drip  from  the  outer  leaf  surface 
of  the  tree.  In  order  to  understand  the  very  important  absorptive 
function  of  roots  it  will  be  necessary  to  discuss  in  some  detail 


FIG.  60.    Root  system  of  the  corn  plant 
(Zea  mays) 

Note  the  uniform  distribution  of  the  roots  in 
the  soil 


STEMS,  ROOTS,  AND  LEAVES 


113 


the  phenomena  of  osmosis  by  living  cells.  It  seems  wiser,  there- 
fore, to  consider  absorption  by  roots  later,  in  connection  with 
the  absorption  and  movement  of  water  in  plants. 


Midveini 


Slamata, 


cells 


LEAVES 

Structure.  Leaves  are  flat  expansions  of  the  plant  body, 
designed  to  present  a  large  surface  of  green  leaf  tissue  to  air 
and  sunlight  for  the  purpose  of  food-making.  The  leafstalk,  or 
petiole,  connects  the  leaf 
blade,  or  expanded  green 
portion  of  the  leaf,  with 
the  conducting  and  storage 
portions  of  the  main  stem. 
It  also  serves  to  display 
the  leaf  effectively  to  sun- 
light and  air.  The  lamina, 
or  blade,  of  the  leaf  (Fig. 
61,  a)  is  composed  largely 
of  green  tissue  cells  called 
mesophyll,  supported  and 
subdivided  into  small  areas 
by  the  intricately  branched 
veins.  The  relation  of 
veins  and  mesophyll  is 
best  appreciated  by  hold- 
ing the  leaf  of  an  ordi- 
nary plant  toward  the 
light.  The  veins  not  only 
form  a  supporting  framework  for  the  leaf  but  also  serve  to 
irrigate  the  mesophyll  cells  with  water  and  to  remove  sugar 
and  other  foods  which  must  be  transported  back  into  the  stem 
from  the  manufacturing  mesophyll  cells.  The  leaf,  like  other 
organs  of  the  plant,  is  therefore  composed  of  layers  and  groups  of 
cells  called  tissues,  each  cell  layer,  or  group,  having  a  special  func- 
tion to  perform  for  the  leaf  and  for  the  entire  body.  The  princi- 
pal leaf  tissues  are  the  epidermis,  the  mesophyll,  and  the  veins. 


FIG.  61.    Gross  structure   of  a  leaf  of   the 
milkweed  (Asclepias) 

a,  surface  and  sectional  views  of  a  milkweed 
leaf;  6,  microscopic  structure  of  the  epidermis 


114  GENEliAL  BOTANY 

The  epidermis  (Fig.  61, />)  is  usually  composed  of  a  single 
layer  of  cells  without  green  chloroplasts,  which  inclose  and  pro- 
tect the  more  delicate  mesophyll  cells  over  the  entire  surface 
of  the  leaf.  Its  cells  are  distinctive  in  that  their  outer  walls  are 
covered  with  a  waxy  secretion  which  makes  them  almost  com- 
pletely impervious  to  water  and  air.  The  epidermis  thus  forms 
an  effective  protection  against  excessive  evaporation  from  the 
mesophyll  cells,  which  would  wither  and  destroy  the  leaf.  The 
lack  of  green  chloroplasts  in  the  epidermis  also  enables  the  sun- 
light to  penetrate  easily  its  translucent  cell  Avails  and  illuminate 
the  green  mesophyll  cells  below  it. 

The  stomata  are  highly  modified  cells  of  the  epidermis  which 
regulate  to  a  certain  extent  the  flow  of  gases  into  the  leaf  and 
the  evaporation  of  water  from  the  internal  air  spaces  between  the 
mesophyll  cells.  Each  stoma  is  composed  of  two  guard  cells 
containing  chloroplastids,  between  which  there  is  a  narrow  slit, 
or  pore,  averaging  about  .008  mm.,  or  ^-Q-Q  of  an  inch,  in  width 
(Figs.  62,  b  and  c).  Although  the  pore  between  the  guard  cells 
is  so  very  small,  ample  provision  is  made  for  the  exchange  of 
gases  with  the  external  air  and  for  the  evaporation  of  water  from 
the  leaf,  since  it  is  estimated  that  the  number  of  stomata  in  the 
epidermis  of  common  leaves  averages  from  100  to  700  per 
square  millimeter.  Stomata  are  usually  open  in  daylight,  when 
the  mesophyll  cells  are  manufacturing  sugar  and  starch,  and  arc 
partially  or  wholly  closed  at  night,  during  the  period  of  leaf 
inactivity.  The  exact  mechanism  for  the  control  of  stomata  is 
not  thoroughly  understood,  although  the  guard  cells  are  known 
to  be  sensitive  to  light. 

The  mesophyll  cells  vary,  in  size,  form,  and  arrangement, 
in  different  kinds  of  leaves.  In  the  more  typical  horizontal 
leaves  (Fig.  62,  a)  the  mesophyll  cells  of  the  upper  surface  next 
to  the  epidermis  are  greatly  elongated,  with  their  long  axes  per- 
pendicular to  the  leaf  surface.  On  account  of  their  palisade- 
like  form  and  arrangement  this  layer  of  mesophyll  cells  is 
termed  the  palisade  layer.  In  vertical  or  erect  leaves,  such 
as  those  of  narcissus  and  many  lilies,  the  palisade  layer  often 
extends  entirely  around  the  leaf  as  a  continuous  layer  beneath 


STEMS,  ROOTS,  AND  LEAVES 


115 


the  epidermis.  The  remaining  cells  of  the  mesophyll  are  called 
spongy  cells,  since  they  are  more  loosely  arranged  than  the 
palisade  layer,  with  large  intercellular  spaces  like  the  canals 
and  pores  of  a  sponge.  This  canal  system  of  intercellular  spaces 
within  the  mesophyll  of  the  leaf  contains  air,  with  water  vapor 
and  gases.  Since  the  composition  of  the  air  within  the  leaf 
differs  from  the  external  air  in  the  relative  amount  of  water 
and  gases  contained  in  it,  we  may  properly  designate  it  as  the 


Epidermis 
./*= 


Palisade  parenchyma 


Midcein 


Spongy  parenchyma  rw 

Gtianl  cell 

Air 


Guard  cells 
C 


FIG.  62.    Microscopic  structure  of  a  leaf  of  the  milkweed  (Asclepias) 

a,  a  transverse  section  of  a  milkweed  leaf;  6,  a  diagrammatie-drawing  illustrating 
the  structure  cf  the  guard  cells ;  c,  the  guard  cells  of  the  milkweed  enlarged 

internal  atmosphere  of  the  leaf.  The  gases  and  water  of  this 
internal  atmosphere  come  in  contact  with  the  external  atmosphere 
outside  the  leaf  through  the  stomata,  with  which  the  intercellular 
spaces  of  the  leaf  are  directly  connected.  All  of  the  mesophyll 
cells  are  also  living  cells,  in  which  green  chloroplastids  are 
embedded  in  a  living  cytoplasmic  sac  surrounding  the  large 
central  water  vacuole.  The  chloroplastids  are  thus  placed  at 
the  outside  of  the  cell,  where  they  are  exposed  to  sunlight  and 
to  the  constituents  of  the  internal  atmosphere.  The  importance 
of  these  internal  arrangements  of  the  leaf  will  be  more  fully 
appreciated  when  we  consider  its  function  in  starch  manufacture 
and  in  the  evaporation  of  water. 

The  veins  are  composed  of  two  kinds  of  tissue  cells;  namely, 
supporting  and  conducting  cells.    The  thick-walled  supporting 


116  GENEEAL  BOTANY 

cells  flank  the  veins  above  and  below  to  prevent  the  collapse  of 
the  conducting  cells  and  to  strengthen  the  entire  framework  of 
the  leaf.  The  conducting  cells  are  in  the  form  of  vascular  bun- 
dles, which  are  continuations  of  the  vascular  bundles  of  the 
stem  and  root.  They  have  the  same  general  character  as  those 
of  the  stem,  except  in  the  smaller  veinlets,  where  the  conducting 
tissue  is  reduced  to  a  few  cells.  The  vein  tissue  thus  puts  the 
manufacturing  mesophyll  cells  in  direct  connection  with  the 
main  axis  of  the  plant. 


SECTION  III.    PHYSIOLOGY 
CHAPTER  VII 

NUTRITION  AND  SEASONAL  LIFE  OF  PLANTS 
PHOTOSYNTHESIS  AND  RESPIRATION 

The  student  has  now  become  familiar  with  the  form  of  plants, 
and  with  their  adjustments  to  the  environment.  He  has  also 
studied  the  structure  of  the  organs  and  tissues  of  the  plant  body, 
upon  which  a  proper  understanding  of  its  various  functions  is 
based.  The  following  discussion  will  therefore  be  confined  to  the 
main  physiological  processes  concerned  with  nutrition ;  namely, 
photosynthesis,  respiration,  transpiration,  assimilation,  and  the 
digestion  of  foods. 

PHOTOSYNTHESIS 

If  a  green  leaf  which  has  been  exposed  to  bright  sunlight  for 
a  few  hours  is  bleached  with  alcohol  and  then  tested  for  starch 
with  iodine,  the  green  mesophyll  areas  will  be  tinted  blue  or 
bluish  black.  This  will  be  found  to  be  due  to  minute  starch 
grains,  which  are  formed  in  the  green  chloroplastids  of  the 
mesophyll  cells  during  the  exposure  of  the  leaf  to  the  sunlight. 
The  starch  grains  are  formed  in  such  numbers  throughout  the 
green  tissues  of  the  leaf  that  the  latter  is  tinted,  owing  to  the 
reaction  of  the  iodine  on  the  abundant  starch. 

This  simple  experiment  indicates  the  principal  function  of  the 
leaves,  which  is  to  manufacture  sugar  and  starch  for  the  plant. 
This  sugar  and  starch  then  becomes  a  basis  for  the  manufacture 
of  other  foods,  such  as  the  fats  and  the  nitrogenous  foods,  or 
proteins.  It  is  probable  that  the  leaf  is  the  main  center  for  the 
compounding  of  these  nitrogenous  foods  from  the  sugar  which 

117 


118  GENERAL  BOTANY 

it  manufactures  within  its  green  mesophyll  cells  and  the  nitrog- 
enous salts  which  come  up  to  the  leaf  from  the  roots  in  the 
water  stream.  The  leaves,  through  their  green  cells,  are  there- 
fore the  manufacturing  organs  for  the  other  members  of  the  plant 
body,  which  are  dependent  upon  them  for  nourishment. 

The  term  photosynthesis  means  literally  the  uniting,  or  com- 
pounding, of  substances  by  means  of  light.  When  applied  to 
the  work  of  a  green  leaf  it  signifies  the  making  of  sugar  from 
the  simple  raw  materials,  carbon  dioxide  (CO2)  and  water  (H.2O), 
by  means  of  energy  supplied  to  the  leaf  by  sunlight.  These  raw 
materials  for  photosynthesis  are  supplied  from  the  soil  through 
the  roots  and  stem  and  from  the  air  through  the  stomata.  The 
carbon  dioxide  is  absorbed  by  diffusion  into  the  internal  atmos- 
phere of  the  leaf  from  the  external  air,  and  is  then  taken  up  by 
the  chloroplasts  located  in  the  mesophyll  cells  surrounding  the 
intercellular  spaces  of  the  leaf.  The  water  which  combines  with 
the  carbon  dioxide,  although  ultimately  supplied  by  the  roots, 
is  immediately  absorbed  for  sugar-building  from  the  cell  sap  of 
the  mesophyll  cells.  Since  carbon  dioxide,  like  other  gases, 
tends  to  move  from  a  point  where  it  is  abundant,  or  concen- 
trated, to  a  point  where  it  is  less  abundant,  we  must  picture  it 
as  constantly  flowing  through  the  stomata  into  the  leaf  during 
the  day,  to  take  the  place  of  that  which  is  absorbed  from  the  in- 
ternal atmosphere  of  the  leaf  in  the  intercellular  spaces  by  the 
mesophyll  cells  for  the  making  of  starch  and  sugar.  The  excess 
of  oxygen  which  is  liberated  during  photosynthesis  likewise  dif- 
fuses out  of  the  leaf  or  is  partly  used  in  the  process  of  respira- 
tion, which  goes  on  both  day  and  night  in  the  leaf  as  in  other 
living  parts  of  plants. 

One  of  the  first  products  of  photosynthesis  is  undoubtedly 
sugar,  but  the  excess  of  sugar  produced,  which  is  not  used  by 
the  living  cells  for  growth  and  repair,  is  usually  transformed 
into  starch  within  the  plastids  of  the  leaf  cells  themselves.  The 
excess  of  sugar  and  starch  formed  in  the  leaf  is  later  transported 
in  the  form  of  sugar  into  the  special  storage  tissues  of  the  stem, 
roots,  fruits,  and  seeds.  During  the  day  this  excess  of  starch 
accumulates  in  the  leaf  cells,  as  can  be  demonstrated  by  testing 


NUTRITION  AND  SEASONAL  LIFE  OF  PLANTS    119 

the  leaf  with  iodine,  but  it  is  finally  transported  back  into  the 
other  organs  of  the  plant,  either  for  immediate  use  or  for  storage. 

The  process  of  photosynthesis  is  not  fully  understood,  but  it 
is  supposed  to  involve  the  union  of  carbon  dioxide  and  water 
to  form  carbonic  acid  (CH2O3).  Under  the  influence  of  sunlight 
and  chlorophyll  the  carbonic  acid  is  reduced  to  form  a  com- 
pound, possibly  formaldehyde  (CH2O),  which  is  then  multiplied 
or  condensed  into  sugar  (C6H12O6).  A  part  of  the  original 
molecule  of  carbonic  acid  is  at  the  same  time  given  off  in  the 
form  of  free  oxygen,  which  represents  the  excess  of  that  gas  not 
needed  for  the  building  of  sugar  molecules.  The  reduction  of 
the  carbonic  acid  is  accomplished,  in  some  manner  which  is  not 
fully  understood,  by  the  sunlight  acting  upon  this  substance 
in  the  presence  of  the  green  pigment  (chlorophyll)  of  the  leaf 
plastids.  It  is  estimated  that  under  ordinary  circumstances  this 
decomposition  would  require  the  production  of  energy  "  equiv- 
alent to  1300°  of  heat,"  and  yet  the  green  leaf,  through  the 
agency  of  chlorophyll,  is  able  to  do  this  work  without  high 
temperatures  or  elaborate  machinery. 

The  importance  of  photosynthesis  to  both  plants  and  animals 
can  hardly  be  overestimated,  since  its  first  products,  sugar  and 
starch,  form  the  basic  food  for  plant  and  animal  nutrition.  Not 
only  are  these  products  the  chief  forms  of  reserve  foods  in  the 
special  storage  organs  and  cells  of  plants  but  they  also  function  as 
the  most  important  material  around  which  other  kinds  of  organic 
foods  are  constructed.  Thus,  the  water  stream  from  the  roots 
brings  up  soil  salts  and  deposits  them  in  the  mesophyll  cells  of 
the  leaves.  The  nitrogen,  sulphur,  and  phosphorus  of  these  soil 
salts  is  then  combined  with  the  sugar  molecules  formed  by 
photosynthesis  into  nitrogenous  foods,  such  as  the  gluten  of 
wheat  and  other  forms  of  protein  food  material.  Without  the 
sugar  furnished  by  photosynthesis  this  formation  of  available 
protein  food,  upon  which  both  plants  and  animals  depend  for 
sustenance,  would  not  be  possible.  We  know  also  that  the  fats 
and  oils  of  such  seeds  as  flax,  hemp,  and  castor  beans  are  derived 
from  sugar  in  some  unknown  manner,  and  that  they  are  recon- 
verted into  sugar  and  starch  during  the  germination  of  such 


120  GENERAL  BOTANY 

seeds  and  the  growth  of  the  embryo.  Fats  and  oils,  therefore, 
like  nitrogenous  foods,  are  primarily  derived  from  the  products 
of  photosynthesis.  All  organic  food  for  both  plants  and  animals 
is  thus  composed  of  the  products  of  photosynthesis,  or  of  food 
substances  which  have  been  built  around  these  products.  In  a 
similar  manner  the  skeletal  and  supporting  structures  of  the 
plant  in  the  form  of  cell  walls  are  immediately  derived  from 
sugar  and  starch  formed  by  photosynthesis.  The  cellulose  which 
forms  the  bulk  of  the  cell-wall  substance  of  plant  tissue  is  closely 
related  to  starch  in  chemical  composition  and  is  undoubtedly 
constructed  from  sugar  molecules  by  the  living  protoplasm  of 
plant  cells.  This  cellulose  framework  of  the  plant  body  also 
comprises  the  bulk  of  the  fuel,  in  the  form  of  wood,  coal,  and 
combustible  oils,  upon  which  mankind  depends,  either  directly 
or  indirectly,  for  heat  and  light.  We  see,  therefore,  that  a  large 
part  of  the  food  supply  for  the  organic  world,  the  skeletal  struc- 
tures of  plants,  and  the  energy  derived  from  fuel  in  the  form  of 
light  and  heat  are  dependent  upon  the  process  of  photosynthesis 
by  green  plants. 

RESPIRATION 

The  process  by  which  living  organisms  secure  energy  by  oxi- 
dation for  carrying  on  their  life  activities  is  termed  respiration. 
Unlike  photosynthesis,  respiration  is  not  confined  to  the  cells  of 
the  plant  body  which  contain  green  chlorophyll,  but  takes  place 
in  every  living  cell  of  the  organism.  On  account  of  its  similarity 
to  combustion  the  respiratory  process  is  most  easily  understood 
by  beginning  students  when  compared  with  the  burning  of  wood 
or  coal  in  a  stove  or  a  furnace.  When  thus  compared  it  is  found 
that  respiration  and  combustion  are  alike  in  that  free  oxygen  is 
absorbed  and  energy  is  liberated,  together  with  certain  waste 
products  which  depend  for  their  complexity  upon  the  nature  of 
the  substance  oxidized.  In  the  burning  of  coal  the  oxygen  unites 
directly  with  the  pure  carbon  of  the  coal,  and  the  products  are 
heat  energy  and  carbon  dioxide,  which  may  be  represented  by 
the  following  general  equation  : 

Coal  (C)  +  oxygen  (O2)  =  carbon  dioxide  (CO2)  4-  energy 


NUTRITION  AND  SEASONAL  LIFE  OP  PLANTS    121 

In  the  burning  of  wood  the  cellulose  of  the  wood-cell  walls 
and.  the  stored  sugar,  starches,  and  nitrogenous  foods  are  chem- 
ically more  complex  than  coal.  As  a  consequence  of  this  com- 
plex chemical  character  of  the  substances  oxidized  or  burned 
the  final  products  are  more  complex  than  in  the  burning  of  coal, 
and  iii  addition  to  the  energy  released  we  have  also  carbon 
dioxide,  water,  and  other  substances  given  off  or  thrown  down 
as  by-products.  In  a  somewhat  similar  manner  we  may  picture 
the  respiration  or  oxidation  processes  which  go  on  in  the  living 
cells  of  germinating  seeds  or  in  other  active  cells  of  the  plant 
body  of  a  growing  plant.  This  combustion  of  more  complex 
compounds  may  also  be  represented  by  a  generalized  formula 
as  follows : 

Sugar  4-  oxygen  =  carbon  dioxide  +  water  +  energy 
(C6H1206)  6  (02)  6  (C02)  6  (H20) 

In  the  living  cells  of  plants  the  compounds  which  are  finally 
oxidized  are  very  complex  and  therefore  yield  more  complex 
final  products  than  is  the  case  in  the  oxidation  of  coal  or  wood. 
Some  investigators  believe  that  the  sugars  and  similar,  substances 
are  directly  oxidized  in  living  cells,  much  as  they-are  in  a  piece 
of  wood  containing  these  substances  as  reserve  foods  or  as  con- 
stituents of  cell  walls.  Others  suppose  that  the  protoplasm,  or 
living  substance,  is  gradually  decomposed  during  the  respiratory 
process  and  that  oxygen  plays  an  important  part  in  the  process, 
which  results  in  the  release  of  energy  and  in  the  production  of 
carbon  dioxide,  water,  and  nitrogenous  wastes ;  for  example : 

Proteins  4-  oxygen  .=  carbon  dioxide  4-  water 

-f  nitrogenous  wastes 
4-  energy 

It  appears,  therefore,  that  while  the  energy  released  is  com- 
parable in  combustion  and  respiration,  the  final  waste  products 
are  more  elaborate  in  the  oxidations  which  take  place  in  living 
cells  than  they  are  in  ordinary  combustion,  for  the  reason  that 
more  complex  compounds,  possibly  including  protoplasm  itself, 
are  decomposed  and  partially  oxidized  in  plant  respiration. 


122  GENERAL  BOTANY 

Another  striking  difference  between  the  oxidations  in  the  living 
cells  of  plants  and  animals  and  the  combustion  of  coal  or 
wood  is  found  in  the  temperatures  at  which  the  two  processes 
take  place.  In  animals  respiration  goes  on  at  the  normal  tem- 
perature of  the  animal  body,  which  does  not  exceed  100°  Fahren- 
heit. The  temperatures  at  which  combustions  are  made  possible 
are  known  to  be  much  higher  than  that  at  which  living  matter 
could  continue  to  exist. 

The  above  comparison  of  the  combustion  of  coal  or  wood  and 
respiration  in  living  organisms  may  be  summarized  as  follows : 

Respiration  is  like  combustion  in  that  oxygen  is  necessary 
for  both  processes  and  both  processes  yield  energy  and  carbon 
dioxide,  or  energy,  carbon  dioxide,  and  water,  as  final  by-products. 
It  also  differs  from  combustion  in  the  complexity  not  only  of 
the  compounds  broken  down  but  of  the  waste  products  which 
result  from  the  oxidation  processes. 

The  function,  or  use,  of  respiration  is  much  the  same  as 
combustion  in  an  engine,  in  that  it  liberates  energy  which  can 
be  used  directly  or  can  be  transformed  so  as  to  furnish  power 
for  work  of  various  kinds.  In  the  living  cells  of  seeds,  leaves, 
stems,  or  roots  the  energy  released  by  respiration  is  used  for 
making  new  protoplasm,  for  cell  division,  for  protoplasmic 
streaming,  and  for  other  vital  processes  necessary  to  the  life 
of  the  plant  organism. 

This  energy,  as  the  student  will  recognize,  is  different  in 
origin  and  in  function  from  the  external  energy  absorbed  from 
sunlight  by  the  chlorophyll  and  used  in  photosynthesis  for  the 
building  of  sugar  and  starch.  The  latter  energy  enables  green 
plants  to  make  sugar,  which  forms  the  basic  organic  food  for  all 
plant  and  animal  life,  while  the  energy  released  by  the  respira- 
tory process  is  necessary  for  maintaining  the  vital  processes  of  all 
cells,  whether  they  are  furnished  with  green  chlorophyll  or  not. 

The  mechanism  of  plant  respiration  is  very  different  from  that 
of  animals  in  that  the  plant  is  not  furnished  with  lungs  and  an 
elaborate  blood  system  for  absorbing  and  distributing  oxygen  to 
the  living  tissues  of  the  plant  body.  The  living  cells  which 
we  have  observed  in  wood  rays  and  in  the  wood  proper  are 


NUTRITION  AND  SEASONAL  LIFE  OF  PLANTS    123 

comparable  to  the  living  muscle  and  nerve  cells  in  animals  in  their 
need  for  a  certain  amount  of  oxygen  for  respiration,  but  there 
is  no  definite  circulating  system  in  plants  to  carry  this  oxygen 
to  these  cells  from  the  external  air.  In  plants  the  oxygen  moves 
through  the  intercellular  system,  which  penetrates  to  all  parts  of 
the  plant  body,  by  the  slow  process  of  gas  diffusion.  The  oxygen 
enters  the  plant  through  leaf  stomata  and  through  the  lenticels 
which  we  have  already  observed  in  the  bark  of  twigs  and  stems. 
The  slow  diffusion  of  oxygen  suffices,  however,  for  the  produc- 
tion of  sufficient  energy  for  the  less  active  tissues  of  higher 
green  plants.  In  the  case  of  more  active  plants,  like  bacteria  and 
yeasts,  and  in  the  active  tissues  of  growing  buds  and  flowers, 
plant  cells  often  equal  or  exceed  animals  in  the  energy  of  the 
respiratory  process. 

Comparison  of  respiration  and  photosynthesis.  These  two  vital 
processes  of  plants  are  often  confused,  owing  to  the  fact  that  the 
same  gases  are  involved  in  both  processes  and  that  they  may  go 
on  at  the  same  time  in  one  organ,  as,  for  example,  the  green  leaf. 

Photosynthesis  is  a  food-building  process  in  which  carbon  di- 
oxide is  absorbed  from  the  external  air  by  cells  containing  green 
chloroplasts  and  combined  with  water  to  make  sugar  and  starch. 
During  this  process  the  excess  oxygen,  contained  in  water  and 
carbon  dioxide,  which  is  not  needed  for  making  sugar,  is  liberated 
as  free  oxygen  into  the  intercellular  system  of  the  plant.  This 
absorption  of  carbon  dioxide  and  the  accompanying  liberation 
of  oxygen  can  only  go  on  in  the  daytime,  when  the  chlorophyll 
can  absorb  the  sun's  energy  for  the  photosynthesis  process.  No 
gaseous  exchange,  therefore,  which  is  due  to  photosynthesis  can 
go  on  at  night  or  in  darkness. 

Respiration  is  exactly  opposed  to  photosynthesis  in  its  need 
for  and  use  of  oxygen  and  carbon  dioxide.  In  this  process  oxy- 
gen is  absorbed,  carbon  dioxide  is  liberated,  and  energy  is  formed 
by  all  living  cells  of  the  plant  body,  regardless  of  whether  they 
contain  chloroplasts  or  not.  Furthermore,  oxygen  is  absorbed 
and  carbon  dioxide  is  liberated  by  plant  cells  at  all  times  of  day 
and  night  as  long  as  they  live  and  need  energy  for  maintaining 
their  vital  functions. 


124  GENERAL  BOTANY 

It  will  occur  to  the  student  that  the  oxygen  liberated  by 
photosynthesis  during  the  day  may  be  'used  by  the  green  leaf 
cells  for  respiration,  and  that  the  carbon  dioxide  liberated  in 
respiration  may  likewise  be  built  by  photosynthesis  into  sugar 
and  starch.  While  this  is  true,  it  does  not  change  the  funda- 
mental distinction  between  the  two  processes  as  to  their  nature 
and  use  in  the  living  plant  organism.  In  addition  to  photo- 
synthesis there  are  other  processes,  notably  fermentation  and 
what  is  termed  anaerobic  respiration,  which  are  closely  related 
to  normal,  or  aerobic,  respiration.  These  processes  will  be  con- 
sidered more  appropriately,  however,  in  connection  with  the  life 
of  the  fungi,  in  a  later  chapter. 

DIGESTION  AND  ASSIMILATION 
DIGESTION 

The  nature  of  the  digestive  process  is  the  same  in  both  plants 
and  animals,  and  consists  in  the  conversion  of  foods  from  an  in- 
soluble into  a  soluble  condition  fitting  them  for  circulation  and 
final  assimilation  by  the  tissue  cells  of  the  body.  The  starch 
which  is  stored  in  leaves,  wood  rays,  tubers,  and  seeds  is  a  good 
illustration  of  an  insoluble  food  which  must  be  converted  into 
soluble  sugar  by  digestion  before  it  can  be  circulated  or  used  by 
the  living  cells  of  growing  parts.  In  like  manner  the  gluten  or 
protein  of  wheat  and  the  fat  of  seeds  like  flax  and  the  castor  bean 
are  insoluble  and  unusable  until  they  are  digested  at  the  time  of 
seed  germination  to  form  soluble  proteins  and  fats  for  the  growth 
of  the  embryo.  The  place  where  digestion  occurs  is  very  different 
in  the  higher  plants  and  animals.  Since  there  are  in  plants  no 
specialized  digestive  organs  like  the  alimentary  canal  of  animals, 
digestion  takes  place  in  the  cells  of  storage  organs  in  any  part 
of  the  plant  body  where  reserve  foods  exist.  In  the  mesophyll 
cells  of  leaves  digestion  is  probably  going  on  at  all  times,  since 
starch  formed  by  photosynthesis  is  continually  being  converted 
into  sugar  for  immediate  use  by  the  leaf  cells  or  for  transport 
along  the  veins  and  the  phloem  of  the  stem  to  the  wood-ray  cells 
and  other  storage  tissues  of  the  stem.  In  germinating  seeds 


NUTRITION  AND  SEASONAL  LIFE  OF  PLANTS    125 

digestion  takes  place  in  the  cells  of  the  storage  endosperm  or  in 
the  embryo  itself,  where  reserve  foods  were  laid  down  during 
the  growth  of  the  seed  from  the  ovule.  Digestion  may  therefore 
be  said  to  be  distributed  throughout  the  plant  body,  and  for  the 
most  part  takes  place  within  the  cell  cavities  in  the  higher  plants 
instead  of  being  localized  in  a  special  digestive  tract. 

The  agents  of  digestion  are  the  so-called  enzymes,  or  ferments, 
familiar  to  us  in  the  secretions  of  the  digestive  glands  of  man, 
such  as  the  saliva,  gastric  juice,  and  intestinal  secretions.  In 
plants  these  ferments  are  usually  formed  by  the  protoplasts  in 
the  cells  where  digestion  goes  on  (Fig.  63)  ;  but  in  some  seeds, 
like  the  grasses  and  cereal  grains,  special  glandular  layers  of 
secreting  cells  exist  where  digestive  ferments  are  formed  and 
then  excreted  into  neighboring  tissue  cells  for  digestive  purposes. 

The  method  of  digestion  is  chemical  in  nature  and  consists,  in 
most  cases,  in  adding  water  to  the  insoluble  food  molecules,  thus 
rendering  them  soluble,  as  in  the  conversion  of  reserve  starch 
to  sugar: 

Starch  +  water  -f  diastase  ferment  =  sugar  +  diastase 
(C6H1006)n  H20  (C12H220U) 

In  this  process,  which  is  termed  hydrolysis,  the  exact  r61e  of 
the  ferment  is  unknown,  since  it  is  not  apparently  destroyed  or 
diminished  by  the  process,  as  indicated  in  the  formula. 

ASSIMILATION 

The  conversion  of  foods  rendered  soluble  by  digestion  into 
the  living  protoplasm  of  plant  cells  is  termed  assimilation.  Since 
this  conversion  is  impossible  unless  the  food  is  in  the  proper 
state,  digestion  and  assimilation  are  closely  linked  processes  in 
plant  nutrition. 

SEASONAL  LIFE  OF  ANNUALS,  BIENNIALS,  AND  PERENNIALS 

We  shall  now  attempt  to  apply  to  the  seasonal  life  of  a  few 
common  plants  the  principles  of  nutrition  already  laid  down. 

In  order  to  make  this  application  as  comprehensive  as  possible, 
plants  will  be  selected  which  live  under  quite  different  conditions 


126  GENERAL  BOTANY 

and  thus  have  a  very  different  organization  and  mode  of  life. 
We  shall  thus  secure  not  only  a  summary  of  the  principles  already 
learned  but  also  a  fundamental  study  of  the  relations  which 
plants  sustain  to  their  environments. 

AN  ANNUAL:  THE  GARDEN  BEAN 

The  bean  plant  is  typical  of  the  most  abundant  and  common 
forms  of  plant  life  that  live  in  medium  conditions  on  land.  It  is 
likewise  representative  of  the  so-called  annual  plants,  which  com- 
plete their  life  cycle  in  a  single  season  and  then  die  down,  leaving 
the  seed  as  the  wintering  and  hibernating  structure  to  perpetuate 
the  race  the  next  season.  The  life  of  an  individual  bean  plant 
for  a  season  (Fig.  64)  will  thus  give  us  a  general  idea  of  the 
seasonal  life  and  activities  of  common  annual  land  plants. 

Food  storage.  As  indicated  above,  the  seed  is  the  wintering 
stage  of  the  bean  plant  and  is  composed  of  an  embryo  plantlet 
in  which  is  stored  an  abundance  of  food  for  the  growth  of  the 
embryo  until  it  becomes  self-supporting.  This  food,  however,  is 
stored  up  in  the  bean  in  the  form  of  solid  grains  of  starch  and 
protein.  Fats  are  also  stored  in  a  condition  unsuitable  for  imme- 
diate use  by  the  embryo.  In  order  that  the  growing  embryo 
plantlet  may  use  this  solid  food,  therefore,  it  must  first  be  trans- 
formed into  soluble  foods  by  digestion. 

Digestion  and  respiration.  Digestion  in  the  bean  is  not  very 
different  from  the  same  process  carried  on  in  a  growing  animal 
fed  upon  beans,  whole  or  ground  into  meal.  In  the  case  of  the 
animal  the  digestive  juices  are  poured  into  the  digestive  tract 
and  mixed  with  the  food  in  the  stomach  and  intestine ;  in  the 
bean  seed  the  cells  of  the  cotyledons  in  which  the  food  is  stored 
secrete  the  digesting  substances  or  ferments,  which  transform 
the  solid  starch  and  protein  into  soluble  sugar  and  protein.  The 
fat  is  likewise  ultimately  transformed  into  sugar  before  being 
used  by  the  growing  plantlet.  In  Fig.  63  these  facts  are  graphi- 
cally illustrated  in  connection  with  the  cotyledons  of  a  growing 
bean  seedling.  In  such  a  seedling  the  life  processes  are  unusually 
active,  as  in  animals.  The  reason  for  this  is  that  growth  in  the 


NUTRITION  AND  SEASONAL  LIFE  OF  PLANTS    127 


plant  involves  the  building  of  new  living  substances,  the  forma- 
tion of  new  cells  by  innumerable  cell  divisions,  and  the  expansion 
of  cells  in  the  plant  body  by  means  of  absorbed  water. 

Respiration    is  also    very    active  in   germinating  seeds    and 
growing  plants,  as  can  easily  be  demonstrated  if   germinating 


Transpiration  Respiration  Photosynthesis 


Cortex 

Phloem  (food  path) 
\Xylem  (water  path) 
Pith 


Absorption 

Water 

Sails-- 
Oxygen' 


Respiratiort 


FIG.  63.   The  main  physiological  activities  of  a  bean  plant  illustrated 
diagrammatically 

seeds  or  growing  seedlings  of  beans  are  placed  in  a  thermos 
bottle  under  proper  precautions.  A  thermometer  thrust  among 
them  will  quickly  demonstrate  the  evolution  of  heat,  which  is 
an  index  of  the  respiratory  activity  of  the  growing  plant  parts. 
Photosynthesis  and  migration  of  foods.  The  leaves  as  they 
unfold  begin  to  absorb  carbon  dioxide  and  the  energy  of  sun- 
light. The  roots  also  absorb  water  and  soil  salts,  which  pass  up 


128  GENERAL  BOTANY 

the  ducts  and  are  combined  by  the  leaf  with  the  carbon  dioxide 
from  the  air  to  form  starch,  sugar,  and  nitrogenous  foods.  As 
long  as  the  plant  is  young  and  growing  this  leaf-made  food  will 
be  used  for  immediate  growth,  but  as  the  leafage  increases,  an 
excess  of  food  will  be  formed  daily  over  and  above  that  used 
for  the  immediate  needs  of  the  organism.  This  excess  of  food, 
stored  in  the  leaves  during  the  periocf  of  active  photosynthesis 
by  day,  is  digested  by  the  leaf  cells  at  night  in  the  manner 
already  indicated  for  the  cotyledons  during  seed  germination. 
The  soluble  sugar  and  protein  thus  formed  then  moves  down  the 
phloem  portion  of  the  veins  and  of  the  vascular  bundles  of 
the  stem  of  the  bean  plant  by  the  process  of  osmosis,  which 
is  the  physical  method  of  movement  of  all  soluble  foods  in 
plants.  As  they  move  downward  in  the  phloem  of  the  main 
vascular  bundles  they  are  absorbed  along  the  way  by  the  cortex 
cells  and  also  pass  horizontally  along  the  wood  rays  toward 
the  pith,  where,  as  we  have  seen,  food  is  often  stored.  In  an 
annual  plant,  like  the  bean,  little  food  is  permanently  stored  in 
the  stem  since  it  is  used  mainly  for  seasonal  growth  and  for  the 
production  of  seeds.  As  soon  as  the  bean  flowers  begin  to 
develop  they  form  centers  of  great  activity  in  growth,  especially 
during  the  formation  of  the  pollen  and  the  young  seeds,  or 
ovules.  The  food  stream  then  begins  to  be  diverted  to  the 
flowers,  in  consequence  of  the  growth  activities  going  on  in  the 
developing  anthers  and  ovules.  As  soon  as  fertilization  has 
taken  place  the  young  seeds  begin  to  form  endosperm,  and  this 
process  necessitates  a  constant  supply  of  soluble  sugar  and  pro- 
tein. Since  osmosis  takes  place  from  points  of  greater  concen- 
tration to  those  of  less  concentration  for  any  given  substance, 
the  cells  of  the  growing  cotyledons  of  the  bean  must  needs 
convert  the  sugar  and  soluble  protein  into  insoluble  starch 
and  protein  grains  in  order  to  reduce  the  concentration  of  solu- 
ble foods  in  their  water  vacuoles ;  otherwise  the  flow  of  food 
toward  these  cells  would  soon  cease.  This  conversion  of  soluble 
sugar  into  insoluble  starch  is  done  by  leucoplastids  in  the  cells  of 
the  cotyledons,  while  storage  protein  granules  are  formed  in  the 
general  cytoplasm  of  the  cells.  Fats  and  oils  also  seem  to  be 


NUTRITION  AND  SEASONAL  LIFE  OF  PLANTS    129 


made  from  sugar  in  small  quantities  by  the  general  cell  cyto- 
plasm. When  the  seeds  are  fully  formed  and  stored  with  food, 
they  are  shed  from  the  mother  plant  and  begin  their  long  winter 
rest  preparatory  to  starting  the  new  bean  plant  of  the  next 
season.  When  winter  comes  on,  the  living  substance  of  the 
plant  body  is  killed  by  frost,  and  the  cellulose  framework  falls  to 
the  ground  and  is  gradually  converted  into  carbon  dioxide  and 


Inflorescence 


Seed  (winter  rest) 
Adult  plant 
FIG.  64.   The  seasonal  history  of  the  annual  bean  plant 


Spring  Seedling 

germination      growth 


water  by  fungi  and  by  bacterial  ferments.  The  seeds,  meanwhile, 
are  furnished  with  resistant  seed  coats  and  rest  safely  upon  or 
in  the  soil  until  the  next  spring,  when  they  germinate  and  pro- 
duce a  new  generation  of  bean  plants. 

Assimilation.  The  conversion  of  food  into  living  protoplasm 
is  constantly  taking  place  as  the  bean  plant  grows  and  pro- 
duces new  organs  and  tissues.  This  conversion  of  food  into 
living  protoplasm  is  called  assimilation.  A  considerable  bulk  of 
cellulose  in  the  form  of-  new  cell  walls  is  also  formed  to  serve 
as  the  skeleton,  or  framework,  for  the  mechanical  support  of  the 
living  substance  of  the  plant  body. 


130  GENERAL  BOTANY 

Movements.  During  these  various  internal  activities  the 
external  organs  of  the  growing  plant  are  actively  engaged  in 
adjusting  themselves  to  the  environment ;  but  this  indispensable 
phase  of  the  plant's  life  has  already  been  dwelt  upon  at  suffi- 
cient length  and  need  not  be  reviewed  at  this  point. 

SUMMARY 

We  see,  therefore,  that  during  its  brief  seasonal  life  the  bean 
plant  carries  on  a  complex  series  of  physiological  activities  quite 
comparable  to  those  maintained  by  the  bodies  of  animals  and  human 
beings  (Fig.  63).  Water  and  crude  salts  are  absorbed  from  the  soil 
by  osmosis  and  are  transported  through  specially  differentiated  ducts 
to  the  manufacturing  mesophyll  cells  of  the  leaves.  Carbon  dioxide 
and  oxygen  stream  in  through  stomata,  circulate  through  the  intercel- 
lular spaces,  and  are  absorbed  by  the  living  cells  for  food  construction 
and  respiration.  Food  is  made  by  photosynthesis  and  is  temporarily 
stored  during  the  day  in  .the  leaf  cells ;  this  food  is  then  digested 
at  night  by  active  digestive  ferments  and  is  transported  through  the 
phloem  and  wood  rays  to  points  of  special  activity  in  growth  or  to 
permanent  storage  tissues  in  the  seeds.  Respiration  and  assimilation 
are  always  going  on  in  all  living  tissues  day  and  night,  but  they  are 
especially  active  in  floral  parts  during  the  period  of  their  formation 
and  in  the  germination  of  seeds  in  spring.  The  seeds  are  the  special- 
ized resting  and  wintering  portions  of  bean  plants  which  are  adapted 
to  carry  the  plant  over  inclement  periods  for  which  the  more  delicate 
working  plant  body  is  unfit,  and  in  them,  therefore,  rests  the  assur- 
ance of  a  new  generation  of  bean  plants  in  each  successive  season. 

A  BIENNIAL:   THE  WHITE  SWEET  CLOVER 

If  we  compare  a  biennial  plant  like  white  sweet  clover  with 
that  of  the  annual  bean  plant  described  above,  we  shall  find  the 
seasonal  and  physiological  history  of  the  biennial  quite  different 
from  that  of  the  annual.  The  general  physiological  processes  con- 
cerned with  seed  germination,  the  absorption  and  movement  of 
water,  photosynthesis,  and  respiration  will  be  similar  in  the  two 
plants ;  but  the  seasonal  history  and  the  handling  and  storage 
of  foods  are  quite  unlike  in  the  two,  on  account  of  the  biennial 


NUTRITION  AND  SEASONAL  LIFE  OF  PLANTS    131 

habit  of  the  white  sweet  clover.  In  such  a  plant  the  first  season 
is  devoted  largely  to  the  manufacture  and  storage  of  food,  and 
this  process  takes  place  for  the  most  part  in  the  more  or  less 
fleshy  taproot.  The  processes  concerned  with  the  migration  of 
food  into  this  taproot  and  its  storage  in  specialized  storage  tissues 
are  identical  with  those  already  described  for  the  bean.  The  special 
storage  portions  of  the  root  are  found  in  tho  very  broad  wood  rays 
and  cortex,  which  become  gradually  filled  with  starch  and  protein 


Second 

(Photosynthesis 
and  reproduction) 

FIG.  05.    The  seasonal  history  of  the  biennial  white  sweet  clover  (Melilotus) 

during  the  late  summer  and  autumn,  after  the  plant  has  reached 
its  growth  for  the  season.  During  this  period  also  the  buds  are 
laid  down  on  the  upper  broad  crown  of  the  main  taproot  for 
the  early  growth  of  stems  and  leaves  in  the  spring  of  the  second 
season.  These  buds  and  the  taproot  then  pass  the  winter  in  the 
condition  shown  in  Fig.  65.  The  following  spring,  when  these 
buds  start  to  grow,  wood-ray  cells  convert  their  reserve  food  by 
digestion  into  soluble  sugar  and  protein,  which  move  at  first 
horizontally  along  the  rays,  then  upward  in  the  phloem  into  the 
expanding  buds.  In  these  buds  the  concentration  of  soluble 
foods  in  the  cells  is  kept  low  by  its  constant  conversion  into 
new  cellulose  walls  and  new  protoplasm  for  the  cells  and  tissues 
formed  in  the  growing  leaves  and  internodes.  The  food  streams 


132  GENERAL  BOTANY 

are  thus  able  to  flow  continually  from  the  root  cells  to  the  bud 
cells  until  the  new  stems  and  leaves  are  formed  for  the  season. 
When  these  are  fully  grown,  the  entire  energy  of  the  biennial 
clover  plant  is  devoted  to  the  production  of  flowers,  fruits, 
and  seeds.  It  is  not  necessary  for  us  to  trace  the  physiological 
processes  involved  in  the  formation  and  maturing  of  these 
structures,  since  they  are  exactly  like  those  which  have  already 
been  recounted  in  the  production  of  seed  by  the  annual  bean 
plant. 

When  the  second  season  is  ended,  the  biennial  sweet  clover 
plant  dies  and,  like  the  annual  bean  plant,  intrusts  to  its  seeds 
the  formation  of  new  individuals  with  the  opening  of  the  next 
growing  season.  It  is  thus  seen  that  the  storage  root  and  buds 
carry  the  plant  over  the  first  winter  period  of  its  biennial  exist- 
ence, while  the  seed  with  its  reserve  food  and  embryo  is  the  part 
which  successfully  endures  the  second  winter.  The  advantages 
and  disadvantages  of  the  biennial  habit  as  exemplified  in  the 
clover  will  be  discussed  in  the  summary  following  the  outline 
of  the  life  of  perennial  plants. 

PERENNIALS 

The  plant  body  in  perennial  plants  continues  its  life  from 
year  to  year,  varying  in  the  length  of  its  existence  with  the  kind 
of  plant  and  the  nature  of  the  surroundings.  Since  the  same 
plant  body  continues  to  live  through  several  seasons,  we  should 
expect  that  perennial  plants  would  manifest  distinct  adaptations 
to  seasonal  changes  which  are  not  necessary  in  annuals  and  are 
less  marked  in  biennials.  In  discussing  the  life  of  perennials, 
therefore,  particular  stress  should  be  laid  on  those  characteristics 
which  are  connected  with  the  perennial  habit. 

The  seasonal  life  of  the  herbaceous  perennial  for  the  first  two 
years  is  like  that  of  the  white  sweet  clover.  The  difference 
between  the  two  is  that  the  roots  and  underground  stems  of  the 
perennial,  when  once,  established,  serve  for  storage  and  the  pro- 
duction of  aerial  shoots  for  many  years  instead  of  for  the  second 
season  only  as  in  biennials.  Many  plants  live  from  year  to  year 


NUTRITION  AND  SEASONAL  LIFE  OF  PLANTS    133 


by  means  of  an  underground  stem  which  is  the  perennial  part 
of  the  plant.  Each  year  an  aerial  annual  part,  like  the  stem  of 
the  bean,  is  sent  up  to  construct  food  and  to  produce  flowers  and 
fruit.  During  the  warm  months  of  summer  the  green  leaves  and 
stem  of  this  aerial  part  manufacture  and  distribute  foods  to  the 
perennial  storing 
underground  stem 
and  to  the  de- 
veloping flowers, 
fruits,  and  seeds. 
The  summer  life 
of  these  peren- 
nials, therefore, 
is  like  that  of 
the  annuals  and 
of  the  biennials. 
But  when  cold 
weather  comes  on, 
the  aerial  portion 
of  the  perennial 
dies  down,  and 
the  protected  un- 
derground por- 
tion stored  with 
food  lies  dormant 
until  spring,  when 
it  again  sends  up 
an  aerial  annual 
shoot  for  food-making  and  reproduction.  In  the  case  of  grasses 
some  of  the  old  annual  foliage  often  survives  the  winter,  but  the 
really  effective  aerial  and  annual  leafage  is  produced  each  season 
from  an  underground  stem,  or  rhizome,  which  serves  the  func- 
tion of  storage  and  hibernation  during  inclement  periods. 

In  perennial  woody  plants  (the  trees  and  shrubs  of  the  temperate 
zones)  (Fig.  66)  the  aerial  plant  body  has  become  adapted  to 
changing  seasons  and  differences  in  climate,  so  that  it  does  not  die 
down,  as  in  herbaceous  perennials,  with  each  inclement  seasonal 


FIG.  66.    The  seasonal  history  of  a  perennial  woody 
plant,  the  locust  (Eobinia) 


134.  GENERAL  BOTANY 

period.  This  adaptation  is  secured  by  the  woody  character  of  the 
stem,  by  the  development  of  a  protective  cork  jacket  of  bark,  and, 
except  in  pines  and  their  relatives,  by  the  seasonal  shedding  of 
the  delicate  leafy  portions  of  the  plant  with  the  advent  of  frost 
or  drought.  In  summer  a  woody  plant  is  essentially  a  mesophyte, 
that  is,  it  is  adapted  to  medium  conditions  of  water  and  temper- 
ature, while  in  winter,  or  during  drought  in  dry  regions,  it  par- 
takes of  the  character  of  a  xerophyte,  that  is,  a  desert  plant.  On 
account  of  the  great  size  and  long  life  of  woody  plants,  especially 
trees,  the  seasonal  life  and  the  entire  history  of  such  a  plant  pre- 
sents an  interesting  contrast  to  that  of  the  other  plants  thus  far 
described.  The  following  brief  sketch  of  the  life  of  an  apple  tree 
will  suffice  to  introduce  the  student  to  the  characteristic  life  of 
woody  perennials. 

The  first  five  or  six  years  of  the  life  of  an  apple  tree  are  devoted 
entirely  to  the  building  of  a  massive  trunk  with  its  great  feeding 
and  absorbing  root  system  and  its  extended  branches  for  the  sup- 
port of  an  enormous  leaf  surface.  The  main  trunk  and  branches, 
as  well  as  the  large  roots,  here  become  the  main  storehouses  into 
which  the  excess  of  food  is  annually  passed  and  stored  in  the 
cortex,  the  rays,  and  the  living  cells  of  the  wood  and  phloem. 

Each  spring  digestion  takes  place  in  these  storage  centers, 
and  food  migration  occurs  along  the  usual  channels  to  the  grow- 
ing parts  of  the  tree.  The  wood  rays  supply  food  to  the  cambium 
directly,  the  phloem  carries  food  upward  to  the  swelling  buds 
and  downward  to  the  growing  root  tips,  while  later  movements 
take  place  in  all  directions  to  growing  cells  in  the  phloem  and 
the  wood.  In  this  way  the  apple  tree  expands  its  leafy  shoots 
and  forms  new  roots  each  spring  for  the  season's  work.  When 
the  tree  is  well  established,  reproduction  begins  with  the  annual 
production  of  flowers,  which  results  in  the  formation  of  seeds  and 
fruit.  After  the  reproductive  phase  begins  in  the  life  of  a  tree, 
all  excess  food  produced  in  the  first  months  of  each  season  by  the 
leaves,  and  a  part  of  that  previously  stored  in  the  trunk  and 
branches,  goes  largely  into  the  forming  of  fruit.  In  the  apple 
tree,  as  in  the  sweet  clover,  the  food  streams  are  therefore  diverted 
from  the  storage  tissues  in  the  stem,  the  branches,  and  the  roots, 


NUTRITION  AND  SEASONAL  LIFE  OF  PLANTS    135 

along  the  flower  peduncles,  into  the  ovaries  and  seeds  which  are 
to  form  the  apple  crop  for  the  current  season.  In  addition  to  the 
sugar  and  other  soluble  foods  which  enter  the  fruit  and  seeds 
water  must  also  be  absorbed,  particularly  by  the  rapidly  expand- 
ing cells  of  the  apples,  since  we  know  that  the  enlargement  of  cells 
is  almost  entirely  dependent  upon  water  for  the  growing  water 
vacuoles.  If  the  trees  are  prevented  from  overproduction  of  fruit 
in  any  one  season,  they  may  repeat  essentially  the  same  reproduc- 
tive history  each  season  for  a  number  of  years,  and  this  is  one  of 
the  methods  now  being  used  in  the  scientific  fruit  culture  for 
securing  a  uniform  annual  crop.  If  the  trees  overproduce  in  one 
season,  they  must  rest  for  a  time  in  order  to  store  up  new  reserves 
in  the  trunk  and  branches  for  producing  fruit  in  abundance. 

After  the  fruit  is  matured,  the  apple  lays  up  a  store  of  food 
in  the  usual  manner  for  its  next  season's  spring  growth  of  buds 
and  roots.  The  ripening  of  the  fruit  and  seeds,  and  the  shedding 
of  the  leaves  with  the  advent  of  frost,  close  this  interesting  and 
active  seasonal  life  of  the  apple  tree,  which  may  be  taken  as 
typical  of  the  life  of  our  common  trees  and  shrubs. 

SUMMARY 

We  have  now  seen  that  the  origin  of  annuals,  biennials,  and  per- 
ennials from  the  seed  involves  the  same  essential  physiological  proc- 
esses. These  processes  include  the  active  digestion  and  circulation 
of  reserve  foods,  respiration  for  the  production  of  growth  energy, 
and,  finally,  adjusting  movements  for  the  proper  placing  of  roots, 
stems,  leaves,  and  flowers  in  the  soil,  air,  and  sunlight. 

All  of  these  plants  have  also  an  active  summer  period  during 
which  food  is  manufactured,  flowers  are  produced,  and  fruits  and 
seeds  are  matured.  The  differences  between  annuals,  biennials,  and 
perennials  consist  largely,  therefore,  in  the  length  of  life  of  the 
plant  body  and  the  consequent  necessity  of  adapting  this  vegeta- 
tive body  to  seasonal  and  climatic  changes.  In  this  adaptation  of 
the  plant  to  the  environment,  annuals  have  a  certain  advantage  in 
maturing  their  seeds  in  one  season,  since  the  seed  is  a  favorable 
structure  for  distribution  and  for  withstanding  cold,  drought,  and 
other  environmental  factors  unfavorable  to  plant  life.  The  annual 
death  of  the  plant  body  is  therefore  of  no  consequence  as  far  as 


136  GENERAL  BOTANY 

perpetuation  of  the  life  of  the  species  is  concerned.  The  plant  body, 
being  of  importance  for  the  warm  season  only,  can  consequently  be 
delicate  and  small,  so  that  the  entire  energy  of  the  organism  may 
be  concentrated  on  the  reproductive  structures.  Annuals,  on  the 
other  hand,  are  not  able  to  hold  their  ground  against  such  perennials 
as  dandelions  and  grass,  since  these  plants  retain  a  position  once 
gained  and  spread  out  vegetatively  from  year  to  year.  Annuals 
are  therefore  good  immigrant  plants,  which  find  new  places  and 
occupy  them  temporarily.  They  are  able  to  do  this  by  means  of  their 
seeds,  which  are  produced  abundantly  each  year  and  are  readily 
disseminated  by  wind,  water,  and  animals ;  but  in  the  end  they 
are  usually  crowded  out  of  their  places  by  the  hardier  and  longer- 
lived  perennial  plants.  Biennials  have  an  advantage  in  special- 
izing the  first  season  on  the  production  and  storage  of  a  large 
amount  of  food  and  in  devoting  this  food  storage  during  the  second 
season  to  the  maturing  of  fruits  and  seeds.  The  biennial  habit  is 
especially  adapted  to  regions  with  recurring  dry  and  wet  seasons. 
In  such  localities  the  rainy  season,  which  is  usually  short,  is  suffi- 
cient for  the  production  of  a  new  plant  body  and  the  storage  of  a  rich 
food  supply.  During  the  dry  season  such  plants  lose  their  leaves  and 
hibernate  in  the  form  of  underground  rhizomes,  bulbs,  or  tubers.  Dur- 
ing the  second  rainy  season  flowers,  fruit,  and  seeds  are  produced,  by 
means  of  which  the  species  is  preserved  and  disseminated.  Herba- 
ceous perennials  are  also  adapted  to  such  climatic  changes  as  those 
indicated  above,  and  have  the  additional  advantage  of  the  perennial 
habit.  In  the  temperate  regions  of  the  United  States,  perennials 
also  represent  the  dominant  herbaceous  types,  since  they  easily  adapt 
themselves  to  medium,  dry,  and  wet  situations  and  hold  the  territory 
once  gained  from  the  less  enduring  annuals. 

Trees  and  shrubs,  although  adapted  to  endure  great  variations  in 
the  environment,  are  not  the  equals  of  the  herbaceous  perennial 
grasses  and  allied  plants  in  adapting  themselves  to  wide  ranges  of 
climate  and  soil.  This  is  shown  by  the  fact  that  the  great  deserts,  the 
plains,  and  the  high  mountain  areas  are  not  their  usual  habitats. 
The  prophecy  has  therefore  been  made  that  the  future  vegetation  of 
the  earth  will  be  derived  from  the  herbaceous  perennial  type  of 
plants.  The  tree  type  had  its  origin  in  the  remote  past,  as  our  coal 
deposits  testify.  It  may  have  given  rise  to  the  herbaceous  per- 
ennials of  to-day,  and  it  may  succumb  in  the  future  to  the  younger 
and  more  progressive  herbaceous  perennial  and  its  offspring. 


CHAPTER  VIII    . 

THE  RELATION  OF  PLANTS  TO  WATER 
THE  MECHANISM  OF  ABSORPTION 

OSMOSIS 

The  particles  of  the  soil  from  which  roots  absorb  water  and  soil 
salts  are  surrounded  by  delicate  films  of  water  (Fig.  69)  in  which 
the  dissolved  portions  of  the  soil  necessary  to  plant  life  are  held 
in  very  dilute  solution ;  namely,  from  .0001  to  .03  per  cent. 
In  order  to  understand  the  method  by  which  roots  absorb  this 
soil  water  and  its  dilute  salt  solutions,  the  student  must  first 
understand  something  of  the  laws  of  osmosis,  upon  which  all 
absorption  and  much  of  the  movement  of  fluids  in  the  plant 
depend.  A  simple  experiment  illustrated  in  Fig.  67,  a,  will  serve 
to  give  the  necessary  data  for  understanding  the  application  of 
osmosis  to  the  movement  of  water  and  soil  salts  into  and  through 
the  plant.  Fig.  67  shows  a  parchment  tube  which  is  not  unlike 
a  root  hair  in  its  form  and  in  its  osmotic  properties.  If  now 
the  parchment  tube  has  been  filled  with  a  strong  solution  of 
common  salt  before  being  placed  in  the  distilled  water,  the  results 
of  osmosis  will  shortly  begin  to  be  manifest  to  the  observer.  The 
water  will  be  seen  to  rise  slowly  in  the  glass  tube,  until  a  column 
several  feet  in  height  is  attained.  At  the  same  time  it  will 
be  found,  by  chemical  analysis  or  by  the  taste  of  the  water  in 
the  jar,  that  minute  quantities  of  salt  have  flowed  out  of  the 
parchment  tube  into  the  pure  water. 

In  the  above  experiment  we  have  illustrated  the  essential  facts 
regarding  osmosis,  or  the  diffusion  of  substances  in  solution 
through  an  osmotic  membrane  which  separates  two  solutions  of 
different  composition.  In  such  cases  the  substance  (for  example, 
salt)  dissolved  in  a  liquid  (for  example,  water)  is  called  a  solute, 

137 


138 


GENERAL  BOTANY 


•Glass  tube 


Cork 


and  the  water  in  which  it  is  dissolved  is  termed  a  solvent.  The 
solutes  are  usually  said  to  be  of  a  certain  concentration,  which 
means  the  relative  amount  of  the  substance  dissolved  in  the 
water  (the  solvent)  in  a  unit  of  volume. 

In  the  experiment,  therefore,  if  the  salt  and  water  are  assumed 
to  occupy  equal  portions  of  the  space  in  the  diffusion  shell,  the 
salt  is  of  greater  concentration  inside  the  shell  than  outside  in 

the  distilled  water, 
where  it  would  be 
nil;  and  the  water 
is  greater  in  amount 
per  unit  of  space 
in  the  jar  than 
inside  the  shell. 
Neglecting  for  the 
moment  the  physi- 
cal explanations  for 
the  movements  of 
the  solvent  (water) 
and  the  solute  (salt), 
we  have  seen  that 
each  tended  to  flow 
from  a  point  of 
greater  to  a  point 
of  less  concentra- 
tion, and  this  result 
may  be  taken  as  a 
common  law,  or  tendency,  of  substances  of  a  liquid  nature  and 
of  different  concentration  separated  by  a  membrane.  Since, 
however,  the  parchment  membrane  in  the  experiment  allowed 
the  water  to  pass  in  freely,  being  permeable  to  it,  and  hindered 
the  outgo  of  salt,  the  result  was  a  great  increase  in  the  volume 
of  the  water  in  the  parchment  tube,  and  a  corresponding  increase 
of  pressure  (called  osmotic  pressure)  which  tended  to  overdis- 
tend  the  tube.  As  the  glass  tube  furnished  an  easy  exit  for  the 
water,  and  a  relief,  as  it  were,  from  the  osmotic  pressure  in  the 
parchment  tube,  the  water  rose  against  gravity,  thus  giving  rise 


Solvent 


FIG.  67.    Experiments  in  osmosis  and  root  pressure 

a,  a  diagram  illustrating  an  experiment  in  osmosis ;  6,  a 

diagram  of  an  experiment  illustrating  the  exudation  of 

water  from  a  cut  stem 


THE  RELATION  OF  PLANTS  TO  WATER          139 

to  a  water  column  in  the  glass  tube.  It  is  quite  probable  that 
the  real  cause  of  the  forcible  inflow  of  water  into  the  parch- 
ment tube,  and  of  the  osmotic  pressure  thus  developed,  may  be 
found  in  the  attraction  which  the  wall  of  the  tube  and  the  parti- 
cles composing  the  solute  have  for  the  water  molecules  them- 
selves. We  are  not  here  so  much  concerned,  however,  with  the 
physical  explanation  of  osmosis  as  we  are  with  its  results,  which 
we  need  to  study  in  order  to  understand  the  work  of  the  plant 
in  absorbing  and  circulating  water  and  foods.  These  results,  as 
we  have  indicated  above,  are,  fiest,  the  tendency  of  solvents  and 
solutes  to  equalize  through  a  separating  membrane,  and,  second, 
the  development  of  a  considerable  osmotic  pressure  within  a 
closed  membrane  into  which  an  excess  is  thus  induced  to  flow. 

ABSORPTION  BY  ROOTS 

The  absorption  of  soil  water  and  soil  salts  by  roots  is  governed 
by  processes  very  similar  to  those  indicated  above  in  the  experi- 
ment with  a  parchment  tube.  The  root  hair,  which  is  the  most 
important  absorbing  portion  of  the  root,  is  a  tubular  extension  of 
a  single  epidermal  cell  (Fig.  68).  Like  most  plant  cells  it  is 
furnished  with  a  delicate  cell  wall  and  a  lining  cytoplasmic  sac 
composed  of  living  protoplasm.  The  center  of  the  root-hair  cell 
is  occupied  by  the  water  vacuole,  containing  a  solution  of  organic 
acids,  salts  absorbed  from  the  soil,  and  in  many  instances  sugar, 
all  dissolved  in  the  water  of  the  vacuole.  The  cell  wall  is  per- 
meable to  mosfc  substances,  but  the  cytoplasmic  sac  resembles 
closely  th&  parchment  membrane  of  a  parchment  tube  in  being 
more  permeable  to  the  water  than  to  the  solutes  dissolved  in  it. 
It  differs  from  the  parchment  membrane  in  being  composed  of 
living  substance  and  in  being  thus  able  to  control  to  a  certain 
extent  its  permeability  to  substances  outside  in  the  soil  and  also 
within  its  water  vacuole.  For  instance,  in  sugar  beets  the  cells 
of  the  root  are  able  to  retain  from  14  to  18  per  cent  of  sugar 
within  the  water  vacuoles  of  the  root  cells,  while  no  sugar  exists 
in  the  soil  water  in  which  the  roots  are  bathed.  Beet  roots  at 
the  same  time  allow  minute  quantities  of  soil  salts,  amounting 


140 


GENEKAL  BOTANY 


on  the  average  to  from  .0001  to  .03  per  cent,  to  pass  into  the 
water  vacuoles  of  the  root  cells  from  the  soil  by  osmosis.  In 
general,  absorption  by  root-hair  cells  is  undoubtedly  to  be 
explained  as  an  osmotic  .process  following  the  laws  already  laid 
down  regarding  the  movement  of  solvents  and  solutes  through 
a  parchment  membrane.  The  actual  phenomena  of  absorption 
and  of  movement  of  water  and  soil  salts  through  the  root  will 
be  more  readily  understood  by  reference  to  Fig.  69,  which  indi- 
cates a  portion  of  a  long  section  of  a  root,  showing  root  hairs, 

a  portion  of  the 
cortex,  and  water 
ducts.  The  lower 
root  hair  is  repre- 
sented surrounded 
by  the  soil,  which  is 
made  up  of  soil  par- 
ticles (solid  black), 
water  films  (concen- 
tric lines),  and  air 
(light  spaces  sur- 
rounding the  soil 
particles).  If  now 
nitrogen  in  the  form 
of  a  nitrate  is  in 
solution  in  the  soil 
water  in  greater  concentration  than  it  is  in  the  water  vacuole  of 
the  root  hair,  the  laws  of  osmosis  already  enunciated  will  insure 
the  inflow  of  the  needed  nitrogen  salt  into  the  root-hair  cells,  and 
thence,  by  the  same  physical  law,  into  the  cortex  cells  which 
surround  the  duct.  Water  will  likewise  tend  to  flow  from  the 
soil  water  into  the  water  vacuoles  of  the  root  hair  and  root  cortex 
cells  as  long  as  these  water  vacuoles  contain  more  solutes,  and 
so  less  water  per  unit  of  volume,  than  the  soil  water  outside. 
The  result  will  be  a  continued  flow  of  certain  soil  salts  into  the 
root  hairs  from  the  soil,  and  a  great  pressure  developed  inside 
of  the  root  hair  and  cortex  cells  by  the  forcible  inflow  of  large 
quantities  of  water  into  the  water  vacuoles  of  these  cells.  This 


FIG.  68.   The  structure  of  root  hairs 

A,  a  transverse  section  of  a  root  with  hairs;  B,  a  single 
hair  with  adhering  soil  particles 


THE  RELATION  OF  PLANTS  TO  WATER 


141 


Palisadea  cells       ,Epidermi» 


mini 
cells 


latter  fact  may  be  practically  demonstrated  by  the  student  in  an 
experiment  similar  to  that  illustrated  in  Fig.  67,  b.  If  the  stem 
of  a  proper  plant  be  cut  off  close  to  the  root,  as  in  the  figure, 
and  a  glass  tube  be  fitted  over  the  cut  end  of  the  stump  by 
means  of  rubber  tubing,  water  will  soon  begin  to  well  out  of 
the  ducts,  which  have  been 
opened  by  cutting  the  stem. 
This  water,  in  an  active 
plant  such  as  a  coleus  or 
a  begonia,  will  often  rise 
to  a  height  of  several  feet 
in  a  small  glass  tube,  or  to 
that  of  from  40  to  50  feet  in 
the  case  of  some  trees.  This 
phenomenon  (erroneously 
called  root  pressure)  is  par- 
tially explained  by  osmosis, 
but  the  ultimate  explanation 
is  as  yet  unknown.  In  nature 
the  outflow  of  water  from 
wounds  usually  occurs  in 
the  spring  before  the  leaves 
unfold ;  it  ceases  as  soon  as 
the  leaves  expand  and  begin 
.  active  transpiration. 

The  soil  salts  taken  into 
the  roots  by  osmosis  move 
with  the  water  into  the 
ducts  and  up  the  stem  to 
the  leaves,  where,  as  we 
have  already  noted,  the  salts  are  combined  with  the  sugar,  which 
results  from  photosynthesis,  to  form  the  basic  nitrogenous  foods 
for  the  entire  plant.  Nitrogenous  foods  are  undoubtedly  formed 
also  in  other  living  cells  of  the  plant  body  in  the  same  manner 
^as  in  the  leaves. 


Cytoplasmic  sac 


Ducts 


aier       ~\ .,  >    i 
films  &#  Pa> 


FIG.  69.   The  path  of  water  in  the  plant 

The  lower  portion  of  the  figure  shows  the 
structure  of  the  soil  and  the  relation  of  root 
hairs  to  the  soil  particles  and  to  the  water 
films;  the  upper  portion  illustrates  the  con- 
nection of  the  leaf  tissues  with  the  ducts  of 
the  stem 


142  GENERAL  BOTANY 

TRANSPIRATION  AND  WATER  ASCENT 

Transpiration  is  a  term  used  to  indicate  the  loss  of  water 
from  leaves  and  other  exposed  organs  of  the  plant.  It  differs 
from  the  evaporation  of  water  from  a  free  water  surface  in  that  it 
is  controlled  by  certain  structural  features  of  the  epidermis 
and  bark,  which  greatly  restrict  the  loss  of  water  from  these 
organs.  Thus,  Sachs  estimated  that  a  given  area  of  sunflower 
leaves  evaporated  only  about  half  as  much  water  as  a  similar 
area  of  free  water  surface. 

Transpiration,  like  evaporation,  is  also  controlled  by  external 
conditions  of  the  atmosphere,  such  as  temperature,  humidity,  and 
air  movements.  Although  the  loss  of  water  takes  place  from 
all  exposed  parts  of  plants  by  evaporation,  the  term  transpira- 
tion is  usually  understood  to  apply  to  the  loss  of  water  from 
leaves,  where  the  greatest  amount  of  evaporation  takes  place. 
In  the  following  discussion,  therefore,  leaf  transpiration  will  be 
mainly  considered. 

Leaf  transpiration.  The  same  structural  features  which  we 
have*  already  noted  as  important  in  the  gaseous  exchanges  con- 
cerned with  photosynthesis  and  respiration  are  also  important  in 
transpiration.  The  delicate  mesophyll  cells  of  the  leaf  (Fig.  62,  a) 
are  surrounded  by  a  system  of  intercellular  spaces  which  open 
out  into  the  external  air  through  innumerable  stomata.  There- 
fore the  water  which  is  supplied  to  these  mesophyll  cells  from 
the  veins  tends  to  evaporate  from  their  cell  Avails  into  the  inter- 
nal air  within  the  intercellular  spaces  of  the  leaf,  from  which 
it  diffuses,  like  a  gas,  into  the  external  air  through  the  stomata. 
If  this  evaporation  of  water  is  too  great,  the  leaf  wilts  and  the 
plant  is  in  danger.  It  is  thus  seen  that  the  need  for  structural 
adaptations  in  the  leaf  to  facilitate  gaseous  exchange  is  often  an 
element  of  danger,  since  they  may  also  lead  to  an  undue  loss  of 
water  by  transpiration.  Therefore  leaves  often  effect  a  compro- 
mise in  their  structure  between  the  need  for  gaseous  exchange 
and  that  of  controlling  water  loss  which  might  endanger  the 
life  of  the  plant.  Some  of  the  important  structural  adaptations 
designed  to  control  transpiration  are  the  following: 


THE  RELATION  OF  PLANTS  TO  WATER          143 

Leaves  in  dry  climates,  in  a  location  where  soil  water  is  not 
readily  available,  are  wont  to  be  smaller  than  in  regions  and 
localities  where  water  is  abundant.  This  contraction  of  the 
leaf  results  in  a  diminution  in  size  of  the  intercellular  spaces, 
which  thus  reduces  the  danger  of  excessive  evaporation  into  the 
intercellular  spaces  from  the  mesophyll  cells.  The  outer  walls 
of  the  epidermal  cells  may  also  become  greatly  thickened  and 
coated  over  with  a  waxy  secretion  called  the  cuticle.  This  pre- 
vents all  loss  of  water  except  through  the  stomata.  The  stomata 
in  most  plants  are  also  able  to  limit  the  amount  of  transpiration 
by  effecting  a  closure  when  the  loss  of  water  from  the  mesophyll 
cells  is  not  balanced  by  that  received  from  the  veins.  This 
opening  and  closing  of  the  stomata  is  partly  explainable  on  phys- 
ical grounds,  but  is  not  as  yet  fully  understood.  Then  again 
many  plants,  like  the  mullein,  have  leaves  in  which  the  epidermal 
cells  grow  out  into  a  thick  coating  of  hairs  which  prevent  loss 
of  water  (Fig.  72).  These  are  only  a  few  of  the  innumerable 
structural  devices  for  controlling  the  excessive  loss  of  water  from 
leaves  by  transpiration. 

Control.  The  external  factors  which  control  transpiration  are 
light,  heat,  humidity,  air  currents,  and  the  available  water  in 
the  soil.  Light  affects  transpiration  largely  through  its  control 
of  the  stomata,  which,  as  was  stated  above,  are  usually  open 
during  sunlight  and  closed  in  darkness.  Temperature  plays  an 
important  role  in  water  loss  on  account  of  its  effect  on  the  leaf 
tissues  and  on  the  water  content  of  the  air.  If  the  leaf  tissues 
are  heated  by  the  sun's  rays,  the  result  is  an  increased  evapora- 
tion from  the  mesophyll  cells  and  an  acceleration  of  diffusion 
from  the  internal  atmosphere  of  the  leaf  through  the  stomata 
into  the  external  air.  This  external  air  will  also  take  up  more 
moisture  when  heated  than  when  cool.  These  facts  are  con- 
firmed by  experience  with  plants  grown  in  warm,  dry  living 
rooms  in  the  home,  where  the  greatest  care  must  be  exercised 
to  prevent  them  from  wilting  on  account  of  the  excessive  loss 
of  water.  It  is  a  well-known  fact  also  that  plants  in  humid 
regions  lose  very  little  water  by  transpiration,  on  account  of  the 
high  relative  humidity  of  the  air.  For  the  same  reason  there  is 


144  GENERAL  BOTANY 

very  little  transpiration  during  rain  or  fog,  when  the  humidity  of 
the  air  approaches  100  per  cent,  while  the  transpiration  is  less  on 
a  moderate  day,  with  the  humidity  at  70  per  cent,  than  it  is  on 
a  dry  day,  with  the  humidity  at  50  per  cent.  Air  currents,  by 
removing  the  water  as  fast  as  it  evaporates  from  the  stomata,  are 
also  important  factors  in  causing  excessive  evaporation,  espe- 
cially when  vegetation  is  parched  by  a  combination  of  low  humid- 
ity and  high  winds.  In  this  case  the  external  coatings  of  hairs, 
already  mentioned,  is  an  important  factor  in  preventing  loss  of 
water,  since  it  helps  to  maintain  a  cushion  of  moist  air  over  the 
entire  leaf  surface,  protected  from  evaporation  by  the  hair  layer. 

It  is  evident  also  that  the  amount  of  water  available  from 
the  soil  may  modify  transpiration  from  the  leaves  through  their 
tendency  to  wilt  as  soon  as  evaporating  overbalances  absorp- 
tion, and  so  to  cause  the  closure  of  the  stomata,  with  a  conse- 
quent check  on  transpiration.  It  is  quite  probable  also  that  the 
leaf  cells  have  some  control  over  their  own  loss  of  water  in  a 
vital  way,  although  the  nature  of  this  control  is  only  indicated 
by  recent  experiments,  which  need  elaboration  and  confirmation. 

Water  ascent.  The  path  of  water  ascent  has  already  been  ex- 
plained as  occurring  in  the  great  water  ducts  which  form  a  part 
of  the  conducting  and  supporting  vascular  system  of  the  plant. 
The  forces  necessary  to  accomplish  the  task  of  lifting  water  from 
the  roots  of  tall  trees  to  the  crown  can  best  be  appreciated  after 
a  brief  statement  of  the  volume  of  water  transpired  and  the  rate 
at  which  it  moves  up  the  ducts  in  the  wood  of  plant  stems.  The 
volume  of  water  exhaled  from  the  leaves  of  ordinary  plants  is 
indicated  by  the  rate  of  transpiration  from  the  leaves.  Ganong 
estimates  that  the  average  daily  transpiration  from  a  square  meter 
(10J  square  feet)  of  leaf  surface  is  50  grams  per  hour  in  daylight 
and  10  grams  per  hour  in  darkness.  A  birch  tree  with  200,000 
leaves  is  supposed  to  give  off  from  300  to  400  kilograms  (from 
660  to  880  pounds)  of  water  on  a  hot  day  in  summer.  Sachs 
estimates  that  a  sunflower  plant  the  height  of  a  man  would  evap- 
orate from  800  to  1000  cubic  centimeters  (about  1  quart)  of 
water  from  its  leaves  on  an  average  July  day.  The  rate  at  which 
this  water  moves  up  the  ducts  varies  in  different  plants,  as  the 


THE  RELATION  OF  PLANTS  TO  WATEK          145 

following  figures,  derived  from  the  experiments  of  Sachs,  will 
show.  Sachs  states  that  a  particle  of  water  may  travel  as  much 
as  100  centimeters  (40  inches)  per  hour  in  the  vessels  of  woody 
plants.  He  found  that  in  one  species  of  willow  the  water 
moved  85  centimeters  per  hour,  in  corn  plants  from  30  to  42 
centimeters,  in  the  sunflower  70  centimeters,  and  in  the  grape 
98  centimeters.  With  these  facts  in  mind  we  may  now  turn 
to  the  forces  available  in  the  plant  for  transporting  these  great 
volumes  of  water,  at  the  rates  indicated,  from  the  roots  to  the 
crown  of  woody  and  herbaceous  plants. 

In  herbaceous  plants  it  is  conceivable  that  capillarity,  or  the 
rise  of  water  in  the  ducts  clue  to  root  pressure,  might  accomplish 
the  work  involved  in  water  ascent.  It  has  been  found,  however, 
that  capillarity  is  not  effective  for  this  purpose  in  tubes  as  large 
as  the  ducts  of  our  common  plants,  and  that  root  pressure,  while 
important  in  the  spring,  before  transpiration  begins,  is  practically 
in  abeyance  during  the  periods  of  vigorous  transpiration.  In  tall 
trees  these  same  forces  would  be  much  less  adequate  than  in  the 
smaller  herbaceous  plants.  As  indicated  above,  no  theory  has 
ever  been  advanced  to  explain  satisfactorily  all  the  aspects  of 
water  ascent,  but  there  are  certain  known  physical  factors  which 
should  be  mentioned  as  furnishing  a  partial  explanation  for  the 
phenomena.  These  factors,  taken  together,  constitute  what  is 
often  called  the  cohesion  theory  for  water  ascent.  The  cohesion 
theory  is  based  upon  the  cohesive  power  of  small  columns  of 
water,  which  is  known  to  be  very  great  when  subjected  to  a 
straight  pull  and  is  variously  estimated  at  from  10  to  150  atmos- 
pheres. If  this  cohesion  of  a  water  column  applies  to  the  water 
in  the  condition  in  which  it  exists  in  the  ducts  and  stems  of 
plants,  a  scientific  explanation  of  water  ascent  is  conceivable  on 
this  basis.  The  entire  column  of  water  in  a  tree  trunk  would 
then  act  like  a  rope  or  chain  and  could  be  drawn  upward  as  a 
whole  if  a  sufficient  lifting  force  were  applied  at  the  top.  This 
lifting  force  is  believed  by  the  advocates  of  the  cohesion  theory 
to  exist  in  the  osmotic  suction  of  the  mesophyll  cells  bordering 
on  the  veins  of  a  leaf.  As  these  cells  evaporate  water  into  the 
intercellular  spaces  of  the  leaf  the  cell  sap  becomes  concentrated 


146  GENERAL  BOTANY 

in  them,  and  a  higher  osmotic  pressure  is  thus  developed.  This 
concentration  of  cell  sap  and  the  increased  osmotic  pressure  pro- 
duce a  suction  force  on  the  less  concentrated  water  content  of 
the  adjacent  cells  of  the  veins.  The  result  would  be  the  with- 
drawal of  sufficient  water  from  the  vein  cells  at  the  top  of  the 
water  column  to  supply  the  mesophyll  cells,  which  would  cause 
a  corresponding  deficiency  of  water  at  the  base  of  the  water 
column  in  the  roots.  This  deficiency  in  the  roots  would  then  be 
supplied  by  absorption  from  the  soil  and  by  the  forcible  filtration 
of  water  into  the  ducts  by  the  cortex  cells  of  the  root.  This  is 
in  brief  the  conception  of  the  -cohesion  theory  of  water  ascent, 
which,  while  not  entirely  satisfactory,  has  the  virtue  of  dealing 
with  known  physical  principles.  Two  of  the  physical  principles 
involved  —  namely,  leaf -cell  suction  and  filtration,  due  to  osmotic 
pressure  in  the  cortex  cells  of  the  root  —  are  known  to  be  opera- 
tive in  the  plant.  The  third  principle  of  cohesion,  while  valid  for 
water  columns  in  general,  may  not  be  applicable  when  applied 
to  water  columns  as  they  exist  in  the  ducts  of  plants. 

The  importance  of  water  ascent  in  plants  is  also  a  question  con- 
cerning which  there  is  considerable  difference  of  opinion.  Some 
writers  consider  transpiration  and  water  ascent  as  real  functions 
of  the  plant,  designed  to  supply  water  and  soil  salts  to  the  living 
mesophyll  cells  of  leaves  as  well  as  to  other  cells  of  the  plant 
body.  Others  regard  it  as  a  dangerous  process  which  is  necessi- 
tated by  the  structural  adaptations  of  the  leaf  for  photosynthesis 
and  respiration.  The  great  intercellular  spaces  of  the  leaf  and  the 
stomata  are  certainly  a  menace  to  the  plant  when  considered  from 
the  viewpoint  of  conservation  of  water.  We  shall  doubtless  find 
that  both  conceptions  of  transpiration  are  partly  true. 

ECOLOGICAL  RELATIONS  OF  PLANTS  TO  WATER 

Mesophytes.  The  plant  structures  thus  far  described  are 
those  which  pertain  to  plants  living  under  medium  conditions  of 
moisture  and  temperature.  Such  plants  are  termed  mesophytes, 
and  they  include  the  great  plant  populations  which  inhabit  most 
of  the  temperate  regions  of  the  earth. 


THE  RELATION  OF  PLANTS  TO  WATER 


147 


From  the  above  discussion  it  will  be  evident  to  the  student 
that  the  amount  of  water  available  for  the  use  of  the  plant  must 
exercise  a  profound  effect  upon  its  form  and  structure.  This 
available  water  is  dependent  also,  in  the  case  of  land  plants,  first, 
upon  the  amount  of  water  in  the  soil  available  for  the  roots  and, 
secondly,  upon  the  atmospheric  conditions,  such  as  temperature 


FIG.  70.    Mesophytic  vegetation 

Meadow  vegetation  occuring  in  zones,  —  herbaceous  plants  of  mint  (Monarda)  in 
the  foreground,  scrub  and  woodland  in  the  background.   After  Clements 

and  relative  humidity,  which  determine  the  amount  of  evapora- 
tion of  water  vapor  from  the  leaves.  If  the  amount  of  available 
water  in  the  soil  is  abundant  and  the  atmospheric  conditions  are 
such  as  to  restrict  excessive  evaporation,  a  medium  type  of  plant 
life  is  developed,  which  is  usually  designated  as  mesophytic. 

The  mesophytes  include  the  broad-leaved  forest  trees,  shrubs, 
and  herbs  with  which  we  are  most  familiar,  and  constitute  the 
typical  vegetation  of  the  great  forests  of  the  temperate  and 
tropical  zones  as  well  as  that  of  the  more  productive  lowland  and 


148 


GENERAL  BOTANY 


plain  regions  inhabited  and  cultivated  by  man  (Fig.  70).  These 
so-called  mesophytes  thus  furnish  the  principal  plant  environ- 
ment of  the  civilized  races  of  mankind,  from  which  have  been 
derived  the  main  food,  forage,  and  fuel  plants  which  minister  to 
man's  comfort  and  progress.  The  form,  structure,  and  physiol- 
ogy characteristic  of  the  typical  mesophytes  have  been  sufficiently 
outlined  in  the  preceding  account  of  the  structure  and  function 
of  the  root,  stem,  and  leaf,  and  need  not  be  elaborated  here. 


FIG.  71.   Xerophytic  vegetation 

Desert  vegetation  made  up  principally  of  "  ornamental  cacti."   Photograph 
furnished  by  the  United  States  Department  of  Agriculture 

Xerophytes.  The  traveler  in  desert  regions  of  the  United 
States,  or  in  sandy  areas  like  the  peninsula  of  Florida,  is  at  once 
impressed  with  the  unusual  forms  assumed  by  the  characteristic 
native  plants  of  these  regions.  In  the  desert  conspicuous  forms 
of  plant  life  are  mostly  of  the  contracted,  cactuslike  type,  which 
is  in  strong  contrast  to  the  expanded,  broad-leaved  types  which 
clothe  the  more  productive  mesophytic  areas  inhabited  by  man. 
Plants  of  this  type  are  called  xerophytes  (Fig  71).  The  reason 
for  the  difference  is  obvious  if  we  consider  for  a  moment  the 
statement  made  at  the  outset  of  this  topic,  namely,  that  the 
water  "supply  available  to  the  plant  is  dependent  upon  the  avail- 
able soil  water  and  upon  the  conditions  controlling  evaporation 
in  any  given  plant  habitat.  In  the  American  deserts  during  the 


THE  RELATION  OF  PLANTS  TO  WATER 


149 


dry  season  the  amount  of  available  water  in  the  sandy  or  alkaline 
soil  is  very  small,  and  the  roots,  which  often  extend  to  great 
depths,  are  wholly  unable  to  provide  a  large  volume  of  water 
for  the  aerial  stem  and  leaves.  The  atmosphere  also,  in  these 
regions,  is  dry  and  hot  during  the  long,  dry  season,  and  the  plant 
is  thus  in  danger  of  losing  the  small  amount  of  water  available 
from  the  roots.  Under  these  extreme  conditions  all  plants  of  the 
expanded  mesophytic  type  are  likely  to  be  destroyed,  and  only 


FIG.  72.    Leaves  of  xerophytes  protected  by  hairs  from  excessive 
loss  of  water 

a,  hairs  of  wormwood ;  b,  of  Convolvulus;  c,  of  Elaeagnus.  After  Korner 

those  contracted  xerophytic  forms  survive  which  are  adapted  to 
the  peculiar  conditions  existing  in  the  desert.  These  xerophytic 
desert  plants  are  characterized  not  only  by  their  contracted  form 
and  restricted  leaf  surface  but  by  peculiarities  in  their  structure 
as  well.  The  protective  epidermis  in  such  plants  is  wont  to  be 
coated  with  wax  or  cutin,  or  is  supplied  with  abundant  hairs  to 
protect  the  plant  from  excessive  surface  evaporation.  The  inter- 
nal tissues  are  also  more  compact,  with  fewer  and  smaller  inter- 
cellular spaces  into  which  water  vapor  can  pass  from  the  living 
cells  and  then  into  the  external  atmosphere.  Many  plants  of  this 
character  are  known  also  to  have  dense  cell  sap,  which  enables 
them  to  hold  water  vapor  and  thus  prevent  its  loss  by  evapora- 
tion. These  and  various  other  modifications  looking  toward  the 


150  GENERAL  BOTANY 

conservation  of  water  characterize  the  typical  dry-land  xero- 
phytes  of  desert  regions.  In  regions  like  the  Florida  peninsula 
and  the  coastal  regions  of  the  American  continent  sandy  soil 
and  brackish  salt  water  often  bring  about  a  condition  approxi- 
mating that  of  the  desert,  since  the  roots  of  plants  in  these 
regions  are  unable  to  secure  a  large  amount  of  water  from  the 
dense  soil  solution.  Similar  conditions  exist  in  undrained  fresh- 
water bogs  and  marshes. 

Tropophytes.  The  student  must  not  entertain  the  idea,  from 
the  above  account  of  typical  xerophytes,  that  xerophytes  and 
mesophytes  are  sharply  marked  off  from  each  other  in  all  regions. 
Dry  conditions  obtain  in  most  mesophytic  areas  at  certain  peri- 
ods of  the  year,  and  the  xerophytic  and  mesophytic  areas  often 
graduate  insensibly  into  each  other.  Thus,  our  common  broad- 
leaved  trees  are  typical  mesophytes  during  those  parts  of  the 
year  when  they  are  supplied  with  abundant  moisture  and  when 
temperature  conditions  are  suitable  for  the  development  of  the 
more  delicate  leaves,  flowers,  and  fruit.  When  winter  comes  on, 
however,  the  frozen  soil  restricts  the  absorption  of  water  by  the 
roots,  and  the  frost  makes  it  difficult  for  leaves  and  flowers  to 
survive.  These  tree  mesophytes  then  become  practically  winter 
xerophytes,  in  which  evaporation  is  restricted  and  temperature 
changes  are  modified  by  a  thick  coating  of  bark  on  trunks  and 
branches  (Fig.  25).  The  buds  of  trees,  likewise,  in  temperate  and 
arctic  regions,  are  adapted  to  the  xerophytic  conditions  of  winter 
and  are  protected  by  the  highly  modified  (indurated  and  hairy 
or  resinous)  bud  scales  common  .in  oaks,  poplars,  and  evergreens. 

Herbaceous  plants  likewise  hibernate  largely  underground  in 
the  form  of  xerophytic  storage  roots  or  stems,  while  the  more 
delicate  mesophytic  aerial  parts  die  down  with  the  advent  of 
frost  and  are  reproduced  each  year  at  the  beginning  of  the 
warm  season.  It  will  thus  be  seen  that  some  plants  have 
become  typical  dry-land  plants  and  retain  permanently  a  xero- 
phytic habit  and  structure.  Others,  which  are  called  tropo- 
phytes,  adapt  themselves  to  the  changing  seasons  and  take  on 
alternately  a  typically  mesophytic  or  xerophytic  form  which  is 
adapted  to  the  water  supply  at  a  given  seasonal  period. 


THE  RELATION  OF  PLANTS  TO  WATER 


151 


The  graduation  of  a  mesophytic  vegetation  into  a  xerophytic 
one  is  epitomized  in  many  mountainous  regions  as  one  ascends 
from  the  base  to  the  summit  of  a  high  mountain.  At  the  base 
and  along  the  watercourses  of  the  mountain  streams  in  such 
regions  typically  mesophytic  conditions  often  prevail,  in  which 
broad-leaved  plants  are  the  dominant  type.  As  one  ascends  to 


FIG.  73.   The  effect  of  exposure,  slope,  and  moisture  on  vegetation 

"  North  slope  covered  with  dense  forest  of  fir  (Pseitdotsuga) ;  exposed  south  slope, 
with  oak  scruh  and  pine."   After  Clements 

higher  altitudes  the  sterility  of  the  soil,  combined  with  the  lack 
of  water  and  the  drying  effect  of  winds,  produces  a  distinct 
type  of  xerophytic  plants,  which  differ  markedly  from  true 
desert  xerophytes.  The  high-altitude  xerophytes  of  mountain- 
ous regions  are  usually  either  herbs  with  thick,  leathery  leaves 
or  low,  straggling  shrubs  and  dwarfed  trees.  In  the  typical 
pine  forests  which  frequently  inhabit  mountain  slopes  (Fig.  73) 
one  notices  that  the  more  resistant  and  hardy  species  are  found 
high  up,  near  the  tree  lines,  while  less  hardy  forms  clothe  the 


152  GENERAL  BOTANY 

lower  slopes  and  the  borders  of  streams.  It  is  thus  often  pos- 
sible to  see  on  a  single  mountain  slope  all  gradations  between 
mesophytes  and  xerophytes,  and  to  gain  a  more  definite  idea 
of  the  factors  which  control  the  great  mesophytic  and  xero- 
phytic  plant  associations  of  the  earth's  surface. 

Hydrophytes.    Hydrophytes  are  plants  which  are  wholly  or 
partially  submerged  in  ponds,  lakes,  and  streams  and  are  thus 


FIG.  74.    Hydrophytic  vegetation 

Pond  lilies,  with  mesophytic  vegetation  in  the  background.   From  Bergen  and 
Caldwell's  "  Practical  Botany  " 

subjected  to  conditions  very  different,  as  regards  water  supply, 
from  those  on  land.  Like  the  typical  xerophytes,  they  are  sub- 
jected to  extreme  conditions,  which  profoundly  affect  their  form 
and  internal  organization.  In  a  typical  submerged  hydrophyte, 
like  the  Elodea  or  the  pond  weed  (Potamogetori),  the  stem  and 
leaves  are  of  a  very  delicate  nature,  since  the  plant  is  protected 
by  its  surrounding  water  medium. 

Partially    submerged    water    plants,    including   water   lilies, 
pickerel  weed,  and  some  grasses  and  sedges,  partake  more  nearly 


THE  RELATION  OF  PLANTS  TO  WATER          153 

of  the  character  of  mesophytes.  There  are,  therefore,  as  in  the 
case  of  xerophytes,  all  gradations  between  true  submerged 
hydrophytes  and  mesophytes.  This  is  readily  observed  along 
the  borders  of  streams  and  lakes,  where  the  vegetation  is  often 
divided  into  more  or  less  clearly  defined  zones  (Fig.  74). 
In  the  water  will  be  found  both  floating  and  attached  hydro- 
phytes of  a  typical  character,  while  along  the  shore  line  a  zone 
of  amphibious  plants  may  graduate  into  the  typical  grasses, 
sedges,  and  cat-tails  of  a  marshy  area.  Farther  back  from  the 
stream  or  lake  typical  lowland  mesophytes  often  pass  insensibly 
into  grass  and  tree  associations,  inhabiting  the  drier  hills  or 
uplands  bordering  the  water  and  marsh  areas.  The  limitations 
of  the  text  will  not  admit  of  a  more  extended  treatment  of  the 
wonderful  and  interesting  adaptations  in  the  form  and  structure 
of  plants  to  the  water  conditions  of  the  soil  and  the  atmosphere. 
Enough  has  been  said,  however,  to  indicate  the  profound  effect 
of  water  as  a  factor  in  producing  the  types  of  vegetation  which 
inhabit  the  various  climatic  regions  of  the  earth's  surface.  The 
problem  of  the  way  in  which  water  as  a  causal  agent  has  been 
able  to  mold  plant  life  is  one  for  the  students  of  variation,  adapta- 
tion, and  evolution  to  solve.  We  know  simply  that  the  fittest 
plants  for  each  particular  water  environment  in  any  given  habitat 
have  been  selected  for  survival  in  the  struggle  for  existence. 


SECTION  IV.    REPRODUCTION 
CHAPTER  IX 

VEGETATIVE  AND  SEXUAL  REPRODUCTION 

Reproduction  is  a  general  term  used  to  designate  the  various 
processes  by  which  a  parent  organism  gives  rise  to  new  organ- 
isms, called  offspring  or  children.  Reproduction  is  fundamen- 
tally a  cellular  process  and  consists  in  all  cases  in  the  separation 
of  single  cells  or  cell  masses  from  a  parent  or  parents,  which 
possess  the  power  of  growth  and  differentiation  to  form  new 
individuals.  Two  distinct  kinds  of  reproduction  occur  in  the 
higher  plants,  which  are  designated  as  vegetative  and  sexual  re- 
production. In  vegetative  reproduction  the  cell  masses  which 
give  rise  to  new  organisms  are  usually  much  less  highly  special- 
ized than  in  single  sex  cells  which  unite  in  sexual  reproduction. 
The  results  of  the  two  processes  are  also  quite  different  in  their 
nature  and  importance  to  man,  as  we  shall  observe  in  the 
discussion  which  follows. 

VEGETATIVE   REPRODUCTION 

Vegetative  reproduction  in  the  higher  plants  takes  place 
exclusively  by  means  of  vegetative  structures.  These  may  be 
parts  of  ordinary  roots,  stems,  and  leaves. or  they  may  be  highly 
modified  parts  of  the  plant  body,  represented  by  bulbs,  tubers, 
corms,  etc. 

STEMS 

One  of  the  simplest  forms  of  vegetative  reproduction  is  that 
of  budding  and  layering,  in  which  ordinary  buds  and  shoots  form 
the  starting  points  for  the  production  of  new  individuals.  In  the 
black  raspberry  (Fig.  75)  and  the  strawberry  (Fig.  76),  layering 

154 


VEGETATIVE  AND  SEXUAL  REPRODUCTION      155 


is  a  natural  process,  since  ordinary  brandies  in  the  black  raspberry 
and  specialized  runners  in  the  strawberry  take  root  and  form  new 


Hooting  branch 


arcnt  plant 


0/spring 

FIG.  75.   Vegetative  reproduction  in  the  black  raspberry 
The  tip  of  a  branch,  or  cane,  has  taken  root  and  formed  a  new  plant 

plants.    This  natural  process  is  imitated  artificially  by  man  in 
propagating  many  plants,  such  as  the  gooseberry,  grape,  etc. 


FIG.  76.    Vegetative  reproduction  by  runners  in  the  strawberry  (Fragaria) 
1,  mother  plant ;  2,  3,  daughter  plants ;  r,  runner ;  b,  bud  on  the  runner 

Multiplying  branches  are  often  formed,  as  in  common  wild 
plantain  (Fig.  77).  In  such  instances  the  branches,  which  are  at 
first  connected  by  the  mother  stem  axis,  may  become  separated 


156 


GENERAL  BOTANY 


by  mechanical  injury  or  by 
decay,  leaving  two  separated 
plants  instead  of  one.  Such 
vegetative  methods  result  in 
clusters  of  new  plants,  often 
seen  in  the  case  of  dandelions 
in  lawns  and  gardens. 

One  of  the  most  familiar  ex- 
amples of  vegetative  reproduc- 
tion by  highly  modified  stems 
is  the  tuber  of  the  common 
potato  (Fig.  78).  The  potato, 
as  the  figure  shows,  is  merely  a 
greatly  swollen  portion  of  an 
underground  stem,  in  which 
the  buds,  or  eyes,  retain  the 
power  of  growth  into  new  plants 
when  placed  under  proper  con- 
ditions. Since  each  eye  can 
form  a  new  plant  if  it  remains 
in  contact  with  some  of  the 
stored  food  within  the  cells  of 

the  tuber,  it  is  readily  seen  that  a  very  rapid  multiplication  may 

result  by  this  means.    In  cultivation  it  is  the  practice  to  cut  the 

tuber  into   several  pieces, 

each  bearing  one  or  more 

buds,    thereby    increasing 

the  output  of  plants  from  1 1  Root 

single  tubers.    In  the  bulb 

(Fig.  79)    and    the    corm 

(Fig.  80)     the     stem     is 

greatly  shortened  and  the 

leaves  are  highly  modified 

scales.     Bulbs    and    corm 

thus     resemble     ordinary 

buds  in  which  the  stem  axis 

ceases  to  grow  in   length     pIG.  73.  Production  of  tubers  in  the  potato 


FIG.  77.    Multiple  stems  in  plantain 


VEGETATIVE  AND  SEXUAL  EEPKODUCTION  157 


spring 


and  either  becomes  distended  with  reserve  food  or  serves  as  an 
attachment  for  scalelike  leaves  and  roots.  Buds  spring  from  the 
axils  of  the  scale  leaves  as  they  do  from  the  leaves  of  an  ordi- 
nary branch,  and 
these  grow  into 
new  bulbs  or  new 

Parent  buib.,,^^  \\ .  /AX.  WareH.1.  corms.        Tubers, 

corms,  and  bulbs 
are  favorite  repro- 
ductive structures 
of  plants  in  dry 
regions  or  in  cli- 
mates where  a  dry 
season  prevails  for 
a  portion  of  the 
year.  When  the 

^7   SeaSOll   COmeS 

oil  the  aerial  2Teen 
,     .       &. 

part  of  the  plant 
dies  down,  and  the  underground  bulb,  tuber,  or  corm  is  able  to 
live  without  perceptible  injury  from  drought.  These  fleshy  stems, 
with  great  stores  of  food,  have  been  changed  and  improved  for 
man's  use  by  high  cultivation 
and  selection.  In  nature  their 
production  illustrates  the  abil- 
ity of  plants  to  adapt  them-  J®  (\^Parent  corm- 

selves  to  various  environmental 
conditions  by  variation  and 
selection. 


FIG.  79.    Bulbs  of  the  garden  tulip 

a,  surface  view  of  a  large  bulb,  showing  the  origin  of 

smaller  bulbs  in  the  axils  of  bulb  scales ;  b,  sectional  view 

of  the  same  bulb,  showing  stem,  roots,  scales,  and  a  lateral 

bulb  (offspring) 


a,   surface  view   of   a   large   corm   with 

smaller  corms;    6,  sectional  view  of  the 

same  corm 


TT  A  "\7TTQ 

FIG.  80.    Corm  of  gladiolus 
Ordinary    leaves,    such    as 
those  of  the  cultivated  begonia 
of  the  greenhouses,  may  repro- 
duce new  plants  vegetatively  when  properly  treated.   The  leaves 
are  usually  cut  and  placed  in  moist  sand,  when  new  plants  spring 
from  the  cut  surfaces  of  the  veins  by  budding.    The  buds  take 


158 


GENERAL  BOTANY 


root  and  in  a  few  weeks  may  be  severed  from  the  propagating 
leaf  as  new,  independent  plants.    In  the  walking  fern  (Camptoso- 

rus  rJdzophyllus) 
(Fig.  81)  new 
plants  are  formed 
from  the  ends 
of  leaves,  which 
bend  downward, 
touch  the  soil  at 


O/spring 


FIG.  81.   The  walking  fern  (Camptosorus) 


their    tips, 
take  root. 

ROOTS 


and 


The  figure  shows  how  new  fern  plants  take  their  origin  from 
the  tips  of  leaves 


Many  ordinary 
roots  may  be 
made  to  repro- 
duce vegetatively  in  a  manner  quite  similar  to  that  outlined 
above  for  the  begonia  leaf.  On  the  other  hand,  roots,  like  stems, 
may  become  highly  modified  for 
vegetative  reproduction.  Com- 
mon examples  of  this  are  the 
roots  of  the  dahlia  and  of  the 
sweet  potato  (Fig.  82),  which, 
like  the  bulb  and  the  tuber, 
are  storehouses  of  reserve  food 
for  the  growth  of  the  young 
plants  which  spring  from  them 
vegetatively. 

In  all  these  cases  of  vegetar 
tive  reproduction  the  offspring 
resemble  the  parent  very  closely, 
since  the  cells  which  produce  the 

new  plants  by  growth  are  all    FlG'  82'  Enlar«ed  edl'ble  roots  of  the 

.  °  sweet  potato  (Ipomoea) 

derived   irom  a  single   parent. 

This  is  often  a  very  distinct  advantage  to  man,  since  it  enables 
him  to  perpetuate  a  favorable  set  of  characters  in  a  new  fruit 
or  vegetable  much  more  easily  than  could  be  done  by  sexual 


VEGETATIVE  AND  SEXUAL  REPRODUCTION      159 

reproduction.  It  is  well  known  that  most  of  the  finest  fruits 
are  now  propagated  vegetatively  by  budding  or  grafting,  and 
seedless  fruits  are  necessarily  perpetuated  in  this  manner. 

Such  plants  often  lose  the  power  of  sexual  reproduction  after 
long  cultivation  and  propagation  by  vegetative  means.  It  is  evi- 
dent from  the  few  examples  of  vegetative  reproduction  indicated 
above  that  the  higher  plants,  much  more  largely  than  the  higher 
animals,  retain  the  power  of  reproduction  by  the  cells  in  all  parts 
and  organs  of  the  body.  This  is  probably  due  to  the  fact  that 
plants,  quite  unlike  animals,  have  a  long  period  of  growth  and 
are  able  to  renew  each  year  the  leaves  and  flowers  of  the  season 
and  to  increase  the  length  of  roots  and  stems  throughout  life. 

SEXUAL  REPRODUCTION 

Sexual  reproduction  differs  from  vegetative  reproduction  in 
the  following  important  particulars.  The  reproductive  cells  in 
sexual  reproduction  are  not  the  ordinary  unmodified  cells  of  the 
plant  body  which  serve  for  vegetative  reproduction,  but  are  rather 
highly  specialized  naked  cells  termed  gametes.  These  gamete 
cells  fuse  to  form  a  new  double  cell,  the  zygote,  and  the  zygote 
produces  a  new  plant  by  cell  division.  When  the  zygote  cell  is 
formed  by  the  union  of  the  two  gamete  cells,  male  and  female, 
derived  from  different  parents,  the  new  organism  which  grows 
from  the  zygote  is  quite  certain  to  be  unlike  either  parent,  since 
it  inherits  through  the  male  and  female  gametes  two  sets  of  hered- 
itary characters.  The  sexual  process,  therefore,  instead  of  produc- 
ing offspring  like  a  given  parent,  as  in  vegetative  reproduction, 
is  quite  certain  to  produce  a  variety  in  offspring.  In  nature  the 
production  of  new  kinds  of  organisms  is  apparently  advantageous 
to  any  given  species,  or  kind,  of  plant  in  meeting  the  require- 
ments of  a  changing  environment  and  the  struggle  for  existence 
to  which  all  organisms  are  subjected.  Some  of  the  new  kinds  of 
offspring  resulting  from  sexual  union  are  quite  certain  to  have 
new  and  advantageous  combinations  of  characters  which  will 
enable  their  possessors  to  win  out  in  the  battle  of  plants  for 
food  and  light  Man  has  taken  advantage  of  this  tendency  in 


160 


GENERAL  BOTANY 


the  offspring  resulting  from  the  sexual  process  to  produce  new 
plants  which  are  either  pleasing  on  account  of  their  beauty  or 
are  useful  for  food,  forage,  or  fuel. 

Other  advantages  have  been  attributed  to  the  sexual  process 
besides  those  which  accrue  to  a  species  by  the  production  of  a 
varied  offspring,  but  these  are  as  yet  unproved. 

In  the  higher  plants  the  male  gametes  are  produced  in  the  so- 
called  pollen  tube,  which  is  an  outgrowth  from  the  pollen  grain. 
They  are  small,  naked  cells  with  a  conspicuous  nucleus  and  a 


Egg  cell 


Seedling  plant 
FIG.  83.    Sexual  reproduction  and  doubling  of  the  chromosomes  in  fertilization 

very  thin  sheath  of  cytoplasm.  The  end  of  the  pollen  tube  rup- 
tures when  it  reaches  the  vicinity  of  the  egg  in  the  ovule,  and 
frees  the  male  gametes.  The  female  gamete  is  larger  than  the 
male  gamete  and  is  furnished  with  a  more  conspicuous  nucleus 
and  a  larger  amount  of  cytoplasm. 

When  the  two  naked  gamete  cells  come  together,  the  male 
and  female  nuclei,  called  pronuclei,  approach  each  other  and 
finally  unite  to  form  a  new  double  nucleus,  —  the  conjugate 
nucleus,  or  fusion  nucleus.  This  process  of  fusion  of  male  and 
female  sex  cells  is  called  fertilization  (Fig.  83). 

The  zygote  which  results  is  a  double  cell  structurally  and 
functionally ;  hence  the  embryo  and  the  young  plantlet  in  the 
seed  must  possess  all  of  the  characters  of  the  parents  which 
entered  into  the  zygote  cell  through  the  gametes.  Since  the  male 
gametes  are  produced  in  the  pollen  grains  and  the  female  gamete 
is  deeply  buried  in  the  ovule  of  a  higher  plant,  a  complicated 


VEGETATIVE  AND  SEXUAL  REPRODUCTION  161 

apparatus  is  necessary  in  order  to  make  sure  that  the   male 
gametes  will  reach  the  female  gametes  and  fertilize  them. 

To  insure  this  union  of  the  sex  cells  the  flower  has  been 
evolved,  with  its  complicated  apparatus  for  attracting  insects  and 
for  holding  the  pollen  brought  to  the  stigma  by  wind  or  insects. 

THE  FLOWER  AND  ITS  PARTS 

The  flower  is  the  reproductive  apparatus  of  the  higher  plants, 
designed  to  insure  sexual  union  of  the  gametes  and  the  produc- 
tion of  embryos  in  the  seed.  The  early  history  of  the  flower 
shows  that  it  is  a  modified  shoot  or  bud  in  which  the  parts  have 
been  changed  to  meet  the  needs  of  a  highly  organized  repro- 
ductive apparatus.  No  attempt  will  be  made  here  to  trace  the 
steps  in  the  development  of  the  flower  or  to  give  its  manifold 
variations  in  the  different  orders  of  plants.  We  shall  rather 
study  the  parts  of  a  typical  flower  and  then  endeavor  to  trace 
the  processes  of  pollination  and  fertilization. 

The  simple  flower  of  the  mandrake  (Fig.  84)  will  be  used 
to  illustrate  the  following  general  discussion  of  the  parts  of  a 
typical  flower  and  its  fruit.  See  also  the  flowers  of  the  marigold 
and  the  buttercup  in  Part  III. 

Peduncle  and  receptacle.  Most  flowers  are  borne  on  a  slender 
stalk,  or  peduncle,  wrhich  is  enlarged  at  its  apex  to  form  the 
receptacle.  The  floral  parts  have  their  origin  on  this  receptacle, 
which  corresponds  to  the  apex  of  the  floral  branch. 

The  perianth.  The  perianth  is  usually  composed  of  two  dis- 
tinct parts :  the  calyx,  composed  of  separate  leaflike  parts  called 
sepals,  and  the  corolla,  composed  of  individual  parts  called  petals. 
The  petals  are  frequently  highly  colored  and  constitute  the 
showy  part  of  common  cultivated  and  wild  flowers.  The  calyx 
springs  from  the  receptacle  just  below  the  corolla ;  in  the  man- 
drake it  is  composed  of  six  sepals,  which  fall  off  as  soon  as  the 
flower  opens  from  the.  bud.  In  addition  to  its  function  as  a  flag 
apparatus  to  attract  insects  the  perianth  serves  as  a  protective 
envelope  for  the  essential  organs  of  the  flower,  the  stamens  and 
the  pistil.  In  the  bud  stage  these  organs  are  completely  inclosed 


162 


GENERAL  BOTANY 


in.  the  perianth,  and  many  flowers  retain  for  some  time  the 
power  of  opening  and  closing  the  calyx  and  corolla  in  response 
to  light,  temperature,  and  moisture.  They  are  thus  able  to 
serve  as  a  daily  protection  to  the  essential  organs  during  the 
entire  flowering  period. 

Essential  organs.   The  essential  organs  of  the  flower  (so  named 
for  the  reason  that  they  bear  the  pollen  and  the  ovules,  which  are 


isccnce  line 
\-Anther  sac 


Funiculus^ 


spores 


Boots         a 
FIG.  84.    Habit  of  the  mandrake  (Podophyllum),  with  flowers  and  floral  parts 

a,  arnandrake  plant  with  a  flower;  6,  a  pistil  in  section,  showing  the  origin  of  the 
ovules  on  the  placenta;  c,  an  ovule,  highly  magnified  to  show  its  parts;  d,  a  stamen, 
with  anther,  showing  the  lines  of  dehiscence  and  the  pollen ;  e,  a  transverse  section 

of  the  anther 

necessary  to  the  production  of  seed)  are  the  stamens  and  the  pistil. 
The  stamens  in  the  simple  types  of  flowers  arise  above  the  petals, 
constituting  one  or  more  whorls,  the  number  varying  in  different 
kinds  of  flowers.  Each  stamen  is  composed  of  a  delicate  stalk, 
or  filament,  which  bears  at  its  apex  the  anther,  composed  of  two 
pollen  sacs.  The  pollen  grains  or  spores  are  developed  within 
the  pollen  sacs.  When  the  pollen  grains  are  ripe,  each  anther 
splits  along  two  lines,  called  the  lines  of  dehiscence,  and  the 


VEGETATIVE  AND  SEXUAL  REPRODUCTION      163 

pollen  sacs  gape  open,  thus  enabling  the  pollen  to  escape.  The 
pollen  is  then  free  to  sift  out  and  to  be  deposited  by  the  wind 
or  by  insects  upon  the  stigma  of  the  same  or  a  different  flower. 
The  deposit  of  pollen  on  the  stigma  is  termed  pollination  and  is 
essential  to  fertilization  and  the  setting  of  seed. 

The  pistil  may  be  borne  singly  on  the  receptacle,  as  in  the 
mandrake,  or  there  may  be  a  cluster  of  separate  pistils  in  a  single 
flower,  as  in  the  buttercup.  The  pistil  is  composed  of  a  sac,  or 
flask-shaped  lower  portion,  called  the  ovary,  and  of  a  terminal 
portion,  called  the  stigma.  The  stigma  is  usually  roughened, 
irregular,  or  furnished  with  hairs  or  a  sticky  fluid  for  the 
retention  of  pollen  brought  to  it  during  pollination.  In  many 
cases  the  stigma  is  joined  to  the  ovary  by  a  narrow  neck  called 
the  style.  The  style  is  sometimes  lacking,  and  then  the  stigma 
is  said  to  be  sessile.  The  ovary  bears  the  ovules  on  a  cellular  out- 
growth, or  ridge,  called  the  placenta.  The  ovules  (Fig.  84,  £,  <?) 
develop  into  the  seeds  after  fertilization  (Fig.  86,  c,  <f). 

GAMETOGENESIS,  FERTILIZATION,  AND  DEVELOPMENT 

Gametes,  or  sex  cells.  The  general  facts  concerning  the 
structure  of  the  sex  cells,  or  gametes,  and  their  union  in  fertili- 
zation have  already  been  discussed  under  the  head  of  sexual 
reproduction.  It  remains,  therefore,  to  give  more  in  detail  the 
origin  and  development  of  the  sex  cells  within  the  pollen  grain 
and  ovule,  and  to  explain  more  fully  how  fertilization  is  effected 
within  the  ovule  through  the  agency  of  the  pollen  tube. 

In  describing  the  formation  of  the  gametes  it  will  be  neces- 
sary to  include  the  structure  of  pollen  and  ovules,  in  order  to 
make  clear  the  processes  by  which  the  gametes  are  developed 
within  these  structures. 

The  young  pollen  grain  is  a  single  cell  with  a  thick  cell  wall 
inclosing  a  dense  granular  protoplast  with  a  large  nucleus 
(Fig.  85,  a).  The  thickened  cell  wall,  which  is  designed  to 
protect  the  pollen  grain  against  desiccation,  is  divided  into  a 
thick  outer  layer  and  a  thin  inner  layer.  The  outer  thickened 
layer  is  protective,  and  the  inner  layer  is  important  in  the 


164 


GENERAL  BOTANY 


Stigma, 


formation  of  a  pollen  tube.  The  single  nucleus  of  the  pollen 
grain  (a)  divides  into  two  nuclei  (&),  around  one  of  which  a 
cell  is  organized  which  is  to  form  the  two  male  gametes,  the 
other  nucleus  being  left  free  in  the  cytoplasm.  The  newly 
organized  cell  divides  to  form  the  two  male  gametes  within  the 
pollen  tube  (<?),  and  the  free  nucleus  becomes  what  is  called  the 

tube  nucleus,  which  is  concerned 
with  the  growth  of  the  pollen 
tube.  The  above  changes  may 
be  completed  while  the  pollen  is 
resting  on  the  stigmatic  surface 
after  pollination,  but  the  male 
gametes  may  not  be  completely 
organized  until  the  pollen  tube 
begins  to  grow  down  through 
the  style  just  before  fertilization. 
While  the  above  changes  are 
going  on  in  the  pollen  the  female 
gamete,  or  egg  cell,  is  being 
formed  in  the  ovule.  Sections 
of  young  ovules  and  of  mature 
ovules  at  the  period  of  fertiliza- 
tion (Fig.  86,  a,  I)  show  that  the 
body  of  the  ovule  is  covered 
by  two  integuments  made  up  of 
layers  of  cells  which  inclose  the 
central  body  of  the  ovule  except 
at  one  point,  where  the  ovule  coats  do  not  quite  come  together. 
This  failure  of  the  ovule  coats  to  meet  leaves  a  pore,  called  the 
micropyle,  leading  to  a  large  central  cavity  within  the  mature 
ovule  (Fig.  86,  6)  called  the  embryo  sac.  Within  this  embryo 
sac  the  egg  apparatus  is  formed,  which  is  composed  of  the  egg 
cell,  or  gamete,  and  two  associated  cells,  called  the  synergids. 
Two  free  nuclei,  the  polar  nuclei,  may  also  be  seen  near  the 
center  or  at  one  end  of  the  sac  at  this  time.  The  end  of  the  sac 
opposite  the  egg  apparatus  also  contains  three  cells,  the  antipo- 
dals,  with  which  we  are  not  now  concerned.  After  the  male 


FIGS.  85.    Germination  of  pollen 
and  pollination 

A,   pollen   germination   and   the  male 

gametes ;  B,  diagrammatic  figure  of  the 

pistil  and  anthers,  with  growth  of  pollen 

tube  through  the  style  to  an  ovule 


VEGETATIVE  AND  SEXUAL  REPRODUCTION      165 


Mlcropyle 


Polar 
Egg      nuclei     Antipodal* 


Integuments 


Micropy 


gametes  are  formed  in  the  pollen  grain  on  the  stigma  and  the 
female  gamete  is  developed  in  the  embryo  sac,  the  plant  has 
before  it  the  problem  of  getting  the  gametes  together.  This 
difficulty  has  been  solved  in  the  seed  plants  by  the  production 
of  the  pollen  tube,  which  forms  a  canal  down  which  the  male 
gametes  move  toward  the  egg  located  in  the  embryo  sac.  Very 
soon  after  the  pollen 
grain  is  deposited  on 
the  stigma  its  outer 
coat  ruptures  and  its 
thin  elastic  inner  coat 
begins  to  extend  in 
the  form  of  a  tube, 
the  pollen  tube,  which 
grows  down  into  the 
tissues  of  the  style. 
The  tube  nucleus  and 
the  male  gametes  pass 
out  of  the  pollen  grain 
soon  after  the  pollen 
tube  starts  to  elon- 
gate, and  move  down 
the  tube  with  its 
growth  and  extension 
through  the  style. 


FIG.  86.   Ovules,  fertilization,  and  seed 
of  the  mandrake 

a,  young  ovule  with  contained   spore  (megaspore) ; 

b,  mature  ovule  with  an  embryo  sac  containing  an  egg 
and  synergidae  at  the  left,  two  polar  nuclei  in  the  center, 
and  three  antipodal  cells  at  the  right ;  c,  fertilization 
with  the  entry  of  the  pollen  tube  into  the  embryo  sac ; 


When  the  pollen  tube      d>  the  seed  with  an  embryo  (light)  and  endosperm 

(dotted).  All  diagrammatic 

reaches     the     ovary 

cavity,  it  is  apparently  attracted  by  some  chemical  substance 
secreted  from  the  micropyle  of  the  ovule,  since  it  turns  sharply 
and  makes  its  way  into  this  pore  between  the  ovule  coats. 
When  it  reaches  the  embryo  sac,  it  comes  in  contact  with  the 
egg  apparatus  and  the  egg  cell,  which  always  lies  at  the  base  of 
the  micropylar  canal.  The  pollen  tube  then  ruptures  at  its  thin 
end,  and  the  tube  nucleus  and  male  cells  enter  the  embryo  sac 
in  the  immediate  vicinity  of  the  egg,  as  illustrated  in  Fig.  86,  c. 
Fertilization.  One  of  the  male  gametes,  or  its  nucleus,  then 
unites  with  the  female  gamete  to  form  the  zygote,  or  fertilized  egg 


166 


GENERAL  BOTANY 


-  Roots - 


Fio.  87.    Seedlings  of  the 
mandrake 

Redrawn  from  Holm 


Cotyledon 
-Hypocotyl 


cell.  This  union  of  the  male  and  female  gametes  constitutes  the 
real  act  of  fertilization.  The  nucleus  of  the  second  male  gamete 
unites  with  one  or  both  polar  nuclei ;  at  least,  this  is  what  hap- 
pens in  a  large  number  of  instances 
which  have  been  investigated.  This 
second  union  of  a  male  nucleus  with 
the  polar  nuclei  results  in  the  formation 
of  the  food-reserve  material,  called  the 
endosperm,  which  is  developed  for  the 
purpose  of  nourishing  the  young  embryo 
plant  until  it  becomes  self-supporting. 

Embryo.  The  development  of  the 
zygote  into  the  embryo  takes  place 
immediately  after  fertilization.  The  de- 
tails of  this  process  differ  in  different 
species  of  plants  and  cannot  be  dis- 
cussed here  for  the  mandrake.  The 
embryo  in  the  seed  (Fig.  86,  d)  con- 
sists of  a  stem,  or  hypocotyl,  two  cotyledons,  and  a  plumule. 

The  seed.  The  seed  consists  of  the 
embryo,  the  endosperm,  and  the  integu- 
ments, which  become  transformed  into  the 
hard  seed  coats  of  the  ripe  seed.  The  seed- 
lings, which  result  from  the  germination 
of  the  seed,  are  illustrated  in  Fig.  87. 

The  fruit.  The  fruit  is  the  ripened 
ovary,  in  which  the  walls  and  the  pla- 
centa become  fleshy  and  constitute  the 
edible  fruit  of  the  mandrake  (Fig.  88). 

The  seeds  are  finally  liberated  by  the  decay  of  the  fruit  and 
lie  dormant  until  conditions  favorable  for  germination  occur. 

POLLINATION 

Pollination  is  the  term  used  to  designate  the  transfer  of  pollen 
from  the  anther  of  a  flower  to  the  stigma.  In  this  text,  in  the  dis- 
cussion pertaining  to  pollination,  the  following  terms  will  be  used 
to  discriminate  between  different  kinds,  or  degrees,  of  pollination. 


Old  stigma 


Old  ovary 


•fedicd  of  flower 


FIG.  88.   Fruit  of  the 
mandrake 


VEGETATIVE  AND  SEXUAL  REPRODUCTION      167 

Kinds  of  pollination.  The  term  self-pollination  will  be  used 
to  indicate  the  transfer  of  pollen  from  the  anthers  of  a  given 
flower  to  the  stigma  of  the  same  flower.  Close-pollination  will 
be  interpreted  as  the  transfer  of  pollen  from  the  anthers  of 
one  flower  to  the  stigma  of  another  flower  or  flowers  on  the 
same  plant.  Close-pollination  thus  defined  is  often  designated 
as  cross-pollination ;  but  since  the  practical  effects  of  close- 
pollination  in  plant  breeding  are  usually  different  from  those  of 
cross-pollination  as  defined  below,  it  is  thought  best  to  retain 
the  above  definition  of  close-pollination.  Cross-pollination  will 
be  used  to  designate  all  cases  in  which  the  pollen  from  flowers 
on  one  plant  is  transformed  to  the  stigma  or  stigmas  of  flowers 
on  another  plant. 

Since  pollination  is  essential  in  the  higher  plants  before  fer- 
tilization can  take  place,  it  is  necessary  for  the  perpetuation 
of  any  given  race  or  species  of  plants  which  is  not  adapted  to 
maintaining  itself  by  vegetative  reproduction.  The  researches 
of  Darwin  also  established  the  fact  that  cross-pollination  is  of 
distinct  advantage  to  many  species  in  producing  stronger  and 
better  offspring.  It  is  not  surprising,  therefore,  that  nature  has 
evolved  a  great  variety  of  novel  and  interesting  devices  for  insur- 
ing both  self-pollination  and  cross-pollination  in  flowers.  In  the 
following  section  the  papilionaceous  flowers  of  the  pea  family 
have  been  selected  to  illustrate  some  devices  for  insuring  self- 
pollination  and  cross-pollination. 

Inflorescence  and  pollination.  In  some  species  the  flowers  are 
borne  singly  from  the  axils  of  ordinary  leaves,  but  in  a  large 
number  of  plant  species  the  flowers  are  clustered,  and  such  flower 
clusters  are  termed  inflorescences  (Fig.  90,  c?).  This  flower  clus- 
ter is  evidently  a  modified  branch  system,  in  which  the  central 
axis,  termed  the  axis  of  inflorescence,  corresponds  to  the  central 
stem  of  a  shoot.  The  leaves  have  been  reduced  to  small  bracts, 
and  the  flowers  replace  branches  which  ordinarily  spring  from 
the  axils  of  the  leaves.  This  agrees  with  the  statement  made 
above  that  flowers  are  really  modified  branch  buds.  The  dis- 
tinct advantage  of  such  an  inflorescence  as  that  of  the  locust 
(Fig.  90,  d)  in  securing  pollination  is  easily  understood  if  one 


168  GENERAL  BOTANY 

watches  a  bee  seeking  for  nectar,  or  pollen,  in  its  flowers.  The 
bee  will  be  seen  to  go  rapidly  from  one  flower  to  another  on  the 
inflorescence,  probing  for  nectar  at  the  base  of  each  flower  and 
so  dusting  its  body  abundantly  with  pollen.  When  one  inflor- 
escence is  exhausted,  the  bee  moves  to  another  and  repeats  the 
process.  It  is  quite  evident  that  abundant  close-pollination  will 
thus  be  effected  by  such  a  bee  between  flowers  of  the  same  plant, 
and  that  cross-pollination  will  be  effected  if  the  bee  visits  succes- 
sively inflorescences  borne  on  different  plants.  Moreover,  many 
more  pollinations  will  occur  than  could  possibly  be  secured  if 
the  flowers  were  borne  separately  from  the  axis  of  the  ordinary 
leaves  of  the  plant.  In  discussing  devices  for  insuring  abundant 
pollination  the  inflorescence  is  therefore  of  prime  importance  as  an 
aid  in  securing  frequent  close-pollination  and  cross-pollination. 
The  head  of  the  common  white  and  red  clovers  and  the  large 
flower  clusters  of  the  sweet  pea  and  bean  are  other  familiar  in- 
stances of  inflorescences  in  the  pea  family  which  are  of  advantage 
in  securing  cross-pollination  of  the  flowers  of  these  species. 

Pollination  devices  in  papilionaceous  flowers.  Structure  of  the 
flower.  The  flowers  of  the  pea  family  are  very  highly  specialized, 
and  some  are  adapted  to  self-pollination  and  some  to  cross- 
pollination.  They  are  usually  called  papilionaceous  flowers,  from 
their  fancied  resemblance  to  butterflies  of  the  genus  Papilio.  The 
general  relations  of  the  floral  parts  as  they  appear  in  the 
garden  pea  are  illustrated  in  Fig.  89.  The  perianth  is  com- 
posed of  both  calyx  and  corolla,  each  having  five  parts  desig- 
nated respectively  as  sepals  and  petals.  The  calyx  is  nearly 
regular,  but  the  corolla  is  very  highly  modified  and  irregular. 
The  largest  petal  is  called  the  standard,  since  it  projects  promi- 
nently, like  a  standard,  from  the  rest  of  the  flower.  The  standard 
petal  overlaps  two  lateral  petals,  or  wing  petals,  and  these  inclose 
two  united  keel  petals,  which  together  form  the  boat-shaped  keel. 
In  the  normal  condition  of  the  flower  the  stamens  and  pistil  are 
inclosed  and  concealed  from  view  by  the  keel.  In  Fig.  89,  c, 
a  flower  is  shown  in  which  the  petals  on  one  side,  including 
one  half  of  the  keel,  have  been  removed  so  as  to  expose  the 
stamens  and  the  pistil  in  their  natural  position. 


VEGETATIVE  AND  SEXUAL  REPRODUCTION      169 


The  stamens  are  united  by  the  lower  part  of  the  filaments,  which 
form  a  membranous  sheath,  or  stamen  tube  (d),  enveloping  the 
ovary  like  a  sac.  Nine  stamens  are  usually  thus  united,  leaving 
the  tenth  stamen  free. 

The  pistil  resembles  closely  the  familiar  pod  of  the  garden 
pea  and  is  composed  of  the  inflated  ovary,  the  style,  and  the 
stigma.  The  ovary  forms  the  pod  or  fruit,  and  the  slender 


Fruit 
Wing  petal    Fmieubu 


Ovary 


Keel  petals 
Stigma 

$ 
f   'Style 

Pistil  _% 

h      Cotyledon 

FIG.  89.    Structure  of  the  papilionaceous  flower  of  the  pea  (Pisum) 

a,  flower ;  6,  irregular  petals ;  c,  stamens  and  pistil  exposed ;  d,  relation  of  stamens 
and  stigma ;  e,  pistil ;  /,  fruit ;  g,  seed ;  h,  embryo  and  cotyledons 

style  bends  sharply  at  its  junction  with  the  ovary,  thus  taking 
up  a  position  at  -right  angles  to  the  latter  structure.  The  style 
is  terminated  by  the  somewhat  enlarged  and  roughened  stigmas. 
Mechanism  of  pollination.  The  flowers  of  a  large  number  of 
species  belonging  to  the  pea  family  are  definitely  adapted  to 
securing  either  close-pollination  or  cross-pOllination  through  the 
agency  of  insects  which  visit  these  flowers  for  nectar  or  pollen. 
In  some  instances,  however,  the  flowers,  like  those  of  the  garden 
pea  and  the  sweet  pea,  are  so  constructed  that  self-pollination 
may  habitually  occur.  In  these  species  the  pollen  ripens  in 


170  GENEKAL  BOTANY 

conjunction  with  the  maturing  of  the  stigma,  and  the  anthers  are 
so  placed  that  the  pollen  is  dusted  onto  the  hairs  of  the  stigmatic 
surface  when  the  anthers  dehisce.  Self-pollination  is  thus  in- 
sured, even  though  insect  visitors  are  artificially  excluded.  On 
the  other  hand,  these  flowers  are  admirably  adapted,  in  many 
respects,  to  close-pollination  or  cross-pollination.  The  conspicuous 
standard  serves  as  a  flag  apparatus,  and  the  nectar  is  so  located 
at  the  base  of  the  flower  that  visiting  insects  are  tempted  to 
probe  into  the  flower  for  it.  The  wing  petals  serve  to  support 
the  visiting  insect ;  and  since  they  are  attached  to  the  keel,  this 
structure  is  certain  to  be  depressed  by  the  weight  of  the  insect's 
body.  The  depression  of  the  keel  petals  results  in  the  exposure 
of  the  anthers  and  the  hairs  on  the  style,  which  snap  up  against 
the  insect's  body  through  the  opening  between  the  keel  petals. 
The  hairy  abdomen  of  the  visiting  insect  is  thus  covered  with 
pollen,  which  may  be  borne  to  other  flowers  on  the  same  plant, 
thus  effecting  close-pollination,  or  to  flowers  on  different  plants, 
effecting  cross-pollination.  The  structure  of  the  pea  flower, 
like  that  of  species  which  are  wholly  .unable  to  secure  self- 
pollination,  thus  manifests  remarkable  adaptations  for  securing 
close-pollination  or  cross-pollination.  In  the  two  examples  which 
follow,  selected  from  the  pea  family,  either  close-pollination  or 
cross-pollination  is  assured,  and  self-pollination  is  prevented  by 
the  structure  of  the  flower  and  by  the  relative  positions  of  the 
anthers,  the  style,  and  the  stigma. 

The  locust  and  red  clover.  The  flower  of  the  common  locust 
may  be  used,  in  contrast  with  that  of  the  pea,  as  a  concrete 
example  of  an  elaborate  adaptive  mechanism  in  a  flower  of  the 
pea  family,  designed  to  insure  either  close-pollination  or  cross- 
pollination.  The  general  arrangement  and  shape  of  the  floral 
parts  in  the  locust  flower  are  so  similar  to  those  of  the  pea 
blossom  just  described  that  no  additional  description  is  necessary 
(Fig.  90,  <?).  The  special  devices  for  insuring  close-pollination 
or  cross-pollination  are  concerned,  as  indicated  above,  with  the 
structure  of  the  perianth,  the  peculiarities  of  the  pistil,  and  the 
relations  of  the  anthers  to  the  style  and  the  stigma.  The  style,  as 
in  the  pea  blossom,  bends  so  as  to  make  a  right  angle  with  the 


VEGETATIVE  AND  SEXUAL  REPRODUCTION      171 


ovary  at  its  point  of  junction  with  the  latter.  The  stigma  termi- 
nates the  style  and  is  surrounded  at  its  margin  by  a  circlet  of  hairs 
which  point  obliquely  upward.  Below  the  stigma  is  a  hairless 
space,  about  one 
fourth  of  a  milli- 
meter in  length, 
which  separates 
the  bristles  encir- 
cling the  stigma 
from  the  stylar 
brush.  This  sty- 
lar brush  is  com- 
posed of  a  zone  of 
hairs  on  the  style 
from  one  to  two 
millimeters  long. 
The  bristles  ar- 
ranged in  a  circle 
around  the  stigma 
are  often  called 
protective  bristles, 
since  they  are 
supposed  to  keep 
the  pollen  of  the 
stamens  away 
from  the  stigma 
of  the  same  flower. 
The  hairs  of  the 
stylar  brush  are 
termed  collecting 
hairs,  since  they 

collect  and  hold  the  pollen  which  is  discharged  upon  them  by 
the  early  opening  of  the  adjacent  anthers.  The  stigma,  which 
is  protected  from  its  own  pollen  by  the  protective  circlet  of 
hairs,  is  sticky  and  remains  receptive  to  foreign  pollen  long  after 
the  pollen  from  the  anthers  in  the  same  flower  has  been  shed 
and  removed.  The  wings  and  the  keel  are  yoked  together  and 


FIG.  90.   Black  locust  (Bobinia  pseudo-acacia) 

a,  winter  twig;  b,  section  through  a  lateral  bud;   c,  com- 
pound  leaf;    d,    inflorescence;    e,    flower  enlarged,    with 
stamens  and  pistil  exposed  by  removal  of  part  of  the  corolla ; 
/,  fruit,  or  legume.   After  Otis,  from  "  Michigan  Trees  " 


172  GENERAL  BOTANY 

are  depressed  at  the  same  time  by  a  visiting  bee.  The  anthers 
in  the  locust  remain  within  the  keel  during  insect  visitation 
and  finally  wither  away  and  disappear.  When  an  insect  visits 
a  locust  flower,  it  alights  upon  the  keel  petals  and  the  wings, 
as  in  other  flowers  of  the  pea  family  described  above.  The 
weight  of  the  bee  depresses  the  keel  and  wing  petals,  thus  pro- 
jecting the  stigma  and  the  pollen-laden  stylar  brush  against  the 
hairy  portion  of  the  insect's  abdomen.  By  this  act  the  abdomen 
is  touched  first  by  the  stigmatic  surface  and  later  by  the  pollen 
of  the  stylar  brush.  The  stigmatic  surface  is  thus  protected 
from  its  own  pollen,  while  the  insect's  body  is  well  dusted 
with  pollen  grains.  When  the  insect  visits  another  flower,  he 
is  quite  certain  to  dust  the  stigma  of  the  second  blossom  with 
the  pollen  of  the  first.  If  a  succession  of  flowers  is  visited,  as 
is  nearly  always  the  case,  the  above  operation  is  repeated  over 
and  over  again,  thus  insuring  abundant  crosses  both  on  the 
same  and  on  different  plants. 

On  account  of  the  large  number  of  inflorescences  on  the  same 
tree  in  the  locust,  bees  are  more  likely  to  pollinate  flowers  on 
the  same  tree  than  on  two  different  trees.  Where  groups  of 
locusts  grow  in  proximity,  however,  the  more  effective  cross- 
pollinations  between  flowers  on  separate  plants  is  almost  certain 
to  be  accomplished. 

The  common  red  clover  (Trifolium  pratense')  is  another 
example,  selected  to  illustrate  a  definite  mechanism  in  a  flower 
of  the  pea  tribe  for  insuring  against  self-pollination  by  insects. 
Darwin  states  that  one  hundred  flower  heads  on  plants  of 
red  clover,  protected  by  a  net,  did  not  produce  a  single  seed, 
while  one  hundred  heads  on  plants  growing  outside,  which 
were  visited  by  bees,  yielded  approximately  2720  seeds.  This 
species  is  thus  largely,  if  not  entirely,  dependent  upon  either 
close-pollination  or  cross-pollination  for  the  production  of  seed, 
and  thus  for  self-preservation  and  dissemination.  The  general 
structure  of  the  flower  of  the  red  clover  is  similar  to  that  of 
the  garden  pea  and  locust  already  described,  and  the  parts  are 
similarly  arranged  to  facilitate  the  dusting  of  the  abdomen  of 
visiting  insects  with  pollen. 


VEGETATIVE  AND  SEXUAL  REPRODUCTION  173 

A  very  large  number  of  species  in  the  different  families  of 
flowering  plants  have  their  flowers  adapted  to  cross-pollination 
and  close-pollination.  In  some  cases  these  adaptations  are  effected, 
as  in  the  pea  family,  by  irregularities  in  the  shape  of  the  corolla 
and  by  the  relative  position  of  the  anthers  below  the  stigma.  In 
other  instances  the  anthers  ripen  before  the  maturing  of  the  stigma 
in  the  same  flower,  or  vice  versa,  thus  insuring  against  self-pollina- 
tion. Again,  there  are  some  plants  in  which  the  pollen  of  a 
given  flower  will  not  germinate  and  form  a  pollen  tube  on  the 
stigma  of  the  same  flower,  but  will  do  so  on  stigmas  of  other 
flowers  of  the  same  species.  His  observation  of  these  various  ar- 
rangements led  Darwin  to  suspect  that  cross-pollination  was  in 
some  way  beneficial  to  plants,  and  resulted  in  his  well-known 
experiments  which  established  the  general  theory  of  the  bene- 
ficial results  of  cross-pollination  in  the  vegetable  kingdom.  These 
experiments  and  results  will  be  considered  in  some  detail  in  the 
next  chapter,  after  a  brief  discussion  of  the  terms  commonly 
used  in  plant  breeding. 


CHAPTER  X 

PLANT  BREEDING  AND  EVOLUTION 
CROSSING  AND  HYBRIDIZING 

Pollination,  in  order  to  be  effective  in  the  production  of  new 
and  better  kinds  of  plants,  must  be  followed  by  the  union  of  the 
gametes  in  fertilization  and  by  the  production  of  seed.  It  is  a 
common  usage,  therefore,  in  practical  work,  to  substitute  the 
terms  self-fertilization,  close-fertilization,  and  cross-fertilization 
for  the  corresponding  terms  already  denned,  which  are  used  to 
designate  different  kinds,  or  degrees,  of  pollination.  It  is  of 
course  understood  in  each  case  that  pollination  has  preceded  the 
union  of  the  gametes  in  fertilization. 

Inbreeding  is  a  term  usually  employed  by  horticulturists  and 
practical  breeders  of  plants  to  include  all  cases  of  self-  and  close- 
pollination  and  self-  and  close-fertilization,  since  it  has  been  found 
that  self-fertilization  and  close-fertilization  are  essentially  the 
same  in  their  effect  upon  the  offspring.  In  other  words,  it  makes 
very  little  difference  in  the  character  of  the  offspring  whether 
the  pollen  from  a  given  flower  falls  upon  the  stigma  of  the  same 
flower  or  upon  that  of  a  different  flower  on  the  same  plant. 

Crossing  is  the  term  applied  to  the  processes  of  pollination 
and  fertilization  where  the  pollen  and  the  male  gametes  derived 
from  a  flower  on  one  plant  effect  pollination  of  the  stigma  and 
fertilization  of  a  female  gamete  borne  by  a  flower  of  a  different 
plant  of  the  same  kind,  or  species.  Thus,  if  pollen  derived  from 
a  locust  flower  on  one  tree  is  transferred  to  the  stigma  of  a 
flower  on  another  locust  tree  of  the  same  kind,  or  species,  both  the 
transfer  of  pollen  and  the  resultant  fertilization  are  designated 
by  the  term  crossing.  The  term  crossing,  therefore,  includes  both 
cross-pollination  and  cross-fertilization. 

174 


PLANT  BREEDING  AND  EVOLUTION  175 

DARWIN'S  EXPERIMENTS  IN  INBREEDING  AND  CROSSING 

Darwin  laid  the  foundation  for  all  modern  experimentation 
in  cross-pollination  and  sross-fertilization  of  plants  in  his  book 
on  "  Cross-  and  Self-Fertilization  in  the  Vegetable  Kingdom."  A 
detailed  account  is  there  given  of  painstaking  and  elaborate 
experiments  which  he  performed  on  a  great  variety  of  plants. 
Darwin's  methods  were  essentially  similar  to  those  pursued 
to-day  in  the  breeding  of  plants  by  cross-pollination  and  hybridi- 
zation. Some  flowers  were  either  self -pollinated  (with  their 
own  pollen)  or  close-pollinated  (with  pollen  from  another  flower 
on  the  same  plant).  Others  were  cross-pollinated  by  placing 
the  pollen  from  one  flower  on  the  stigma  of  a  flower  on  a 
different  plant  of  the  same  species.  The  plants  were  then  pro- 
tected from  cross-pollination  by  insects  or  wind  by  means  of  fine 
cloth  stretched  over  frames  so  as  to  cover  the  plants  and  protect 
them  from  insect  visitors.  In  modern  practice  the  anthers  are 
removed  from  the  flowers  on  the  pollinated  plants,  and  they 
are  then  inclosed  in  parchment  or  paper  bags  to  exclude  pollen 
borne  by  insects  or  wind  (Fig.  93). 

After  the  plants  had  formed  their  seeds,  Darwin  collected 
the  seeds  from  the  self-fertilized  and  cross-fertilized  species 
and  estimated  their  relative  number  and  weight  in  all  cases. 
The  seeds  from  the  self -fertilized  and  the  cross-fertilized  plants 
were  then  sown,  and  the  relative  vigor  of  the  offspring  de- 
rived from  the  two  kinds  of  seeds  was  noted  and  accurately 
measured.  Darwin  proved  by  these  laborious  methods  that 
fifty-seven  species,  or  kinds,  of  plants  produced  more  vigorous 
offspring  as  a  result  of  cross-fertilization  than  resulted  from 
close-fertilization  of  plants  belonging  to  the  same  species. 

The  nature  of  Darwin's  methods  and  results  is  so  important 
that  the  following  tables  and  diagram  have  been  selected  from 
his  book,  with  an  extract  from  his  summary  of  results. 

The  first  diagram  and  the  accompanying  conclusions  relate 
to  an  experiment  with  the  common  morning-glory  (Ipomoea 
purpurea),  in  which  ten  generations  of  plants  were  cross- 
pollinated.  The  second  table  and  summary  give  similar  results 


176 


GENERAL  BOTANY 


and  conclusions  concerning  experiments  in  cross-pollinating  and 
self-pollinating  plants  of  the  lupine  (LupinuQ  perennis).  In  this 
case  plants  of  the  second  generation  were  used  for  the  experi- 
ment. In  his  experiments  with  the  garden  pea  (Pisum  sativurti), 
illustrated  in  the  last  table,  Darwin  secured  results  which 
differed  from  those  obtained  in  the  majority  of  plants  with 
which  he  worked.  This,  as  we  shall  see,  is  in  harmony  with 
some  recent  results  obtained  by  East,  Shull,  and  other  scientists. 

Morning-glory.    The  mean  height  of  the  self-fertilized  plants  in 
each  of  the  ten  generations  is  also  shown  in  the  accompanying 

diagram,  that  of  the  inter- 
crossed plants  being  taken  at 
100 ;  and  on  the  right  side 
we  see  the  relative  heights  of 
the  seventy-three  intercrossed 
plants  and  of  the  seventy- 
three  self-fertilized  plants.  The' 
difference  in  height  between 
the  crossed  and  self-fertilized 
plants  will  perhaps  be  best 
appreciated  by  an  illustration  : 
If  all  the  men  in  a  country 
were  on  an  average  6  feet  high, 
and  there  were  some  families 
which  had  been  long  and 
closely  interbred,  these  would 
be  almost  dwarfs,  their  average 
height  during  ten  generations 
being  only  4  feet  8^  inches. 


Diagram  showing  the  mean  heights  of 
the  crossed  and  self-fertilized  plants  of 
Ipomaeapurpurea  in  the  ten  generations, 
the  mean  height  of  the  crossed  plants 
being  taken  as  100.  On  the  right  hand 
the  mean  heights  of  the  crossed  and  self- 
fertilized  plants  of  all  the  generations 
taken  together  are  shown 


It  should  be  especially  ob- 
served that  the  average  differ- 
ence between  the  crossed  and 
self-fertilized  plants  is  not  due 

to  a  few  of  the  former  having  grown  to  an  extraordinary  height  or 
to  a  few  of  the  self-fertilized  having  surpassed  their  self-fertilized 
opponents,  with  the  following  few  exceptions :  the  first  occurred  in 
the  sixth  generation,  in  which  the  plant  named  Hero  appeared ;  two 
in  the  eighth  generation,  but  the  self-fertilized  plants  in  this  genera- 
tion were  in  an  anomalous  condition,  as  they  grew  at  first  at  an 


PLANT  BREEDING  AND  EVOLUTION 


177 


unusual  rate  and  conquered  for  a  time  the  opposed  crossed  plants ; 
and  two  exceptions  in  the  ninth  generation,  though  one  of  these 
plants  only  equaled  its  crossed  opponent.  Therefore,  of  the  seventy- 
three  crossed  plants  sixty-eight  grew  to  a  greater  height  than  the 
self-fertilized  plants  to  which  they  were  opposed. 

Lupine.  When  the  seedlings  were  only  four  inches  in  height,  the 
crossed  plants  had  a  slight  advantage  over  their  opponents.  When 
grown  to  their  full  height,  every  one  of  the  crossed  plants  exceeded 
its  opponent  in  height.  Nevertheless,  the  self-fertilized  plants  in 
all  three  pots  flowered  before  the  crossed.  The  measurements  are 
given  in  the  following  table: 


LUPINUS  LUTEUS 


NUMBER  OF  POT 

CROSSED  PLANTS 

SELF-FERTILIZED  PLANTS 

i  I 

Inches 
33f 
30  1 

Inches 
24f 
18| 

1 
•       I 

30 

28 

n                         { 

29| 

26 

I 

30 

25 

in  . 

30| 
31 

28 

27| 

1 

31| 

24| 

Total  in  inches      .... 

246  1 

201  1 

The  eight  crossed  plants  here  average  30.78  and  the  eight  self- 
fertilized  plants  25.21  inches  in  height,  or  as  100  to  82.  All  of  these 
plants  were  left  uncovered  in  the  greenhouse  to  set  their  pods,  but 
they  produced  very  few  good  ones,  perhaps  in  part  owing  to  few 
bees  visiting  them.  The  crossed  plants  produced  nine  pods,  contain- 
ing on  an  average  three  seeds,  so  that  the  seeds  from  an  equal  num- 
ber of  plants  were  as  100  to  88. 

Garden  pea.  In  1867  I  covered  up  several  plants  of  the  Early 
Emperor  pea,  which  was  not  then  a  very  new  variety,  so  that  it  must 
already  have  been  propagated  by  self-fertilization  for  at  least  a  dozen 
generations.  Some  flowers  were  crossed  with  pollen  from  a  distinct 
plant  growing  in  the  same  row,  and  others  were  allowed  to  fertilize 
themselves  under  a  net.  The  two  lots  of  seeds  thus  obtained  were 


178 


GENERAL  BOTANY 


sown  on  opposite  sides  of  two  large  pots,  but  only  four  pairs  came 
up  at  the  same  time.  The  pots  were  kept  in  the  greenhouse.  The 
seedlings  of  both  lots  when  between  six  and  seven  inches  in  height 
were  equal.  When  nearly  full  grown  they  were  measured  as  in  the 
following  table : 

PI  SUM  'SATIVUM 


NUMBER  OF  POT 

CROSSED  PLANTS 

SELF-FERTILIZED  PLANTS 

I   t  -  .  ' 

Inches 
35 

Inches 
29£ 

„  { 

31| 
35 

51 
45 

I 

37 

33 

Total  in  inches      .     .     .     ; 

138  1 

158  f 

The  average  height  of  the  four  crossed  plants  is  here  34.62,  and 
that  of  the  four  self-fertilized  plants  39.68,  or  as  100  to  115.  So  the 
crossed  plants,  far  from  beating  the  self-fertilized,  were  completely 

beaten  by  them. 

Conclusions.  The  first  and 
most  important  of  the  conclu- 
sions which  may  be  drawn  from 
the  observations  given  in  this 
volume  is  that  cross-fertilization 
is  generally  beneficial  and  self- 
fertilization  is  injurious.  This 
is  shown  by  the  differences  in 
height,  weight,  constitutional 
vigor,  and  fertility  of  the  off- 
spring from  crossed  and  self- 
fertilized  flowers,  and  in  the 
number  of  seeds  produced  by 
the  parent  plants. 


FIG.  91.    Increased  vigor  due  to 
crossing  in  corn 

The  two  small  ears  at  the  right  and  left 
represent  two  pure  strains  of  corn  derived 
hy  continued  self-pollination  and  self- 
fertilization;  the  middle  ear  represents 
the  first-generation  hybrid,  derived  hy 
crossing  the  two  strains.  After  East 


Darwin's  early  experiments 
were  in  many  instances  some- 
what faulty,  but  his  conclu- 
sions have  been  found  to 
be  in  the  main  correct  for 


PLANT  BREEDING  AND  EVOLUTION 


179 


both  wild  and  cultivated  species  of  plants  and  animals.  A 
great  majority  of  cultivated  species  apparently  profit  by  cross- 
pollination,  and  many  species  actually  deteriorate  if  continually 
self-pollinated.  Common  field  corn  has  been  shown  by  the 
experiments  of  East  and  Shull  to  belong  to  this  class  of  plants. 
These  investigators  selected  and  isolated  two  strains  of  corn 
(Fig.  91),  which  were  then  close-pollinated  for  several  generations. 
The  result  was,  in 
each  case,  a  greatly 
weakened  race,  with 
the  ears  reduced  to 
nubbins.  But  when 
these  two  weakened 
races  were  crossed, 
offspring  with  much 
increased  vigor  and 
productiveness  were 
secured.  Any  such 
first-generation  cross 
will  always  split  up 
in  succeeding  gener- 
ations into  a  variety 
of  forms  in  which  the 
characters  of  the  par- 
ents will  be  differ- 
ently combined,  and 
the  importance  of  such  crosses  to  agriculture  and  horticulture  is 
therefore  somewhat  diminished.  In  other  species,  as  in  Darwin's 
experiment  with  peas,  no  advantage  arises  from  crossing,  and 
the  plants  are  evidently  adapted  to  continuous  self-pollination 
and  self-fertilization.  Among  the  species  which  are  known  to 
be  adapted  to  continuous  self-pollination,  without  detrimental 
effects,  are  many  of  the  cereals,  including  wheat,  barley,  and  oats, 
as  well  as  peas,  soy  beans,  potatoes,  tomatoes,  flax,  and  tobacco. 
Darwin's  pioneer  experiments  were,  nevertheless,  of  funda- 
mental importance  in  emphasizing  the  value  of  cross-pollination 
and  cross-fertilization. 


FIG.  92.    Increased  yield  of  first-generation 
crosses  in  corn 

Showing  two  ears  of  Tyler's  White-Cap  Dent  at  the 
left  and  of  Burwell's  Yellow  Flint  at  the  right.  The 
first-generation  cross  of  these  two  varieties  (repre- 
sented by  the  two  ears  in  the  center)  has  consistently 
yielded  more  than  the  dent,  the  higher-yielding  parent. 
Photograph  furnished  hy  the  Connecticut  Agricultural 
Experiment  Station 


180 


GENERAL  BOTANY 


HYBRIDIZATION  AND  THE  PRODUCTION  OF  NEW  VARIETIES 

Hybridization  is  the  crossing  of  plants  of  more  distant  rela- 
tionship than  the  members  of  one  species.     Cross-fertilization 

between     different 

varieties  of  culti- 
vated corn,  wheat, 
barley,  apples,  and 
carnations  results 
in  what  are  known 
as  variety  hybrids. 
Crossings  between 
different  species  of 
plants,  for  example 
those  between  rasp- 
berries and  black- 
berries, yield  what 
are  termed  species 
hybrids.  These  are 
apt  to  be  sterile, 
which  accounts  for 
the  fact  that  of  cul- 
tivated fruits  and 
vegetables  compar- 
atively few  have 
resulted  from  hy- 
bridization. Unless 
the  cross-pollination 
and  fertilization  take 


FIG.  93.    Hybridizing  wheat 


Note  the  operator  with  his  tools :  namely,  a  box  contain- 
ing strips  of  paper  and  pins  for  covering  the  wheat  heads ; 
tags  for  labeling;  alcohol  for  sterilizing  hands  and  in- 
struments ;  forceps  and  scalpel.  The  wrapped  heads  of 
grain  have  already  been  pollinated  and  labeled.  From 
Babcock  and  Clausen's  "  Genetics  in  Relation  to  Agri- 
culture." Photograph  by  William  C.  Matthews 


place  between  plants 
which  are  too  far 
removed  from  each 
other  in  relation- 
ship, the  offspring 
will  usually  inherit  increased  vigor  and  also  display  a  greater 
variety  of  characters  than  is  attainable  by  self-fertilization  or 
close-fertilization.  This  result  is  exactly  what  we  should  expect 


PLANT  BREEDING  AND  EVOLUTION 


181 


from  our  knowledge  of  the  nature  of  the  fertilizing  process.  If 
pollen  from  a  white  flower  is  placed  on  the  stigma  of  a  red  one, 
the  germ  cells  which  unite  to  form  the  zygote  are  going  to 
transfer  to  the  zygote  cell,  and  to  the  embryo  which  grows  from 
it,  the  white  and  the  red  characters  of  the  two  parents.  Exactly 
the  same  thing  may  happen 
with  reference  to  any  of  the 
different  characters  of  two 
parents  which  are  crossed. 
The  offspring  are  certain, 
therefore,  to  inherit  the  double 
set  of  characters  received  from 
the  parents.  Experiment  has 
shown  that  the  parental  char- 
acters may  be  variously  com- 
bined in  the  different  offspring 
of  a  given  cross.  We  are 
familiar  with  all  of  these  phe- 
nomena in  the  human  race, 
where  the  children  in  a  family 
inherit  in  different  ways  and 
in  different  degrees  the  char- 
acters of  their  parents. 

If,  therefore,  a  sufficiently 
large  number  of  crosses  are 
made  and  the  offspring  are 
carefully  observed,  almost 
any  combination  of  parental 
characters  may  be  obtained 
by  the  experienced  plant  breeder.  Two  instances  selected  from 
the  work  of  Burbank  will  suffice  to  illustrate  the  truth  of  this 
statement.  Burbank  selected  a  wild  blackberry,  with  white  or 
cream-colored  fruit,  which  was  too  small  to  be  valuable  for 
eating.  He  crossed  this  white  blackberry  with  a  large  edible 
blackberry,  the  Lawton,  in  an  endeavor  to  secure  a  white  edible 
blackberry.  Among  the  hundreds  of  hybrids  which  resulted 
from  extensive  crossing  a  few  showed  the  desired  combination 


FIG.  94.  A  hybrid  wheat  and  the  parents 

Parents  at  the  right  and  left ;  hybrid  in  the 
center.  The  size  and  awn  characters  of  the 
hybrid  are  intermediate ;  the  grains  and 
covering  bracts  resemble  the  parent  at  the 
left.  Photograph  by  the  Minnesota  Agri- 
cultural Experiment  Station.  From  Bergen 
and  Cald well's  "  Practical  Botany  " 


182  GENERAL  BOTANY 

of  characters  of  the  wild  and  the  cultivated  parents.  Burbank 
was  then  able  by  careful  selection  to  perpetuate  a  new  race 
of  white  edible  blackberries.  He  had  caused  a  reshuffling  of 
parental  characters,  which  resulted  in  a  new  combination  of 
characters. 

In  a  similar  manner  Burbank  set  out  to  secure  a  large  and  pro- 
lific daisy  for  commercial  purposes.  To  secure  the  size,  luster,  and 
hardiness  which  he  desired  he  selected  and  crossed  three  kinds  of 
daisies,  each  of  which  possessed  one  of  the  characters  which  he 
wished  to  combine  in  a  new  daisy.  He  therefore  crossed  a  French 
daisy,  for  its  beautiful  pearly  white  luster,  with  a  large  English 


a  b  c  d 

FIG.  95.    Hybridization  in  plums 

a,  a  stoneless  wild  plum  which  was  crossed  with  an  edible  French  prune ;  b-e, 
offspring  of  the  cross.   Adapted  from  a  photograph  by  Burbank 

daisy  and  with  the  common  prolific  American  daisy.  Luster,  large 
size,  and  abundant  flower  production  were  the  characters  which 
Burbank  sought  to  combine  in  a  single  new  race  of  daisies.  By 
careful  selection  from  the  offspring  of  repeated  crossings  he 
finally  secured  a  new  daisy,  the  Shasta,  in  which  all  of  these 
three  desired  characters  were  combined  in  one  plant. 

In  considering  these  results  of  crossing,  it  should  be  stated  that 
Burbank  did  not  create  or  secure  any  new  characters  in  his  daisies. 
The  union  of  the  gametes  in  his  repeated  crosses  resulted  in  the 
mingling  of  the  characters  of  the  three  daisy  parents  in  the 
zygotes  in  various  combination,  and  in  some  zygotes  the  com- 
binations which  Burbank  sought  chanced  to  come  together. 
When  the  zygotes  grew  into  plants,  they  manifested  in  their 
external  appearance  the  luster  of  the  French  daisy,  combined 
with  the  size  of  the  English  daisy  and  the  abundant  flowering 


PLANT  BREEDING  A.ND  EVOLUTION  183 

habit  of  the  American  daisy.  This  favorable  chance  combination 
was  selected  as  the  Shasta  daisy,  a  well-known  garden  variety. 

The  most  marked  results  from  the  hybridization  of  cultivated 
varieties  have  been  obtained  among  the  ornamental  plants,  which 
are  most  highly  protected  from  competition  and  from  the  adverse 
effects  of  soil  and  climate.  The  brilliant  effects  due  to  the  colors 
of  flowers  and  foliage  among  such  plants  as  the  common  phlox, 
pansies,  tulips,  and  foliage  plants  are  in  most  instances  due  to 
the  mixing  of  races  and  varieties  by  a  long  series  of  hybridiza- 
tions similar  to  those  recounted  above  in  the  production  of  the 
Shasta  daisy  by  Burbank.  Among  fruits  and  vegetables  there 
are  fewer  successful  hybrids  than  among  ornamental  plants. 
Almost  no  important  hybrids  are  recorded  among  garden 
vegetables.  Among  the  fruits  there  are  some  hybrid  grapes 
and  pears,  but  in  apples,  peaches,  plums,  cherries,  and  cur- 
rants there  are  no  important  recognized  commercial  hybrids. 
Among  blackberries  there  are  hybrids  between  blackberries  and 
dewberries,  and  between  the  black  and  white  blackberries  already 
indicated.  Among  raspberries  there  are  hybrids  between  the  red 
and  the  black  varieties.  Perhaps  the  most  notable  hybrids  are 
those  obtained  among  citrous  fruits,  such  as  the  tangelo  (which 
is  a  hybrid  between  the  pomelo  and  the  tangerine)  and  the 
citranges  (which  are  hybrids  between  the  sweet  orange  and  the 
hardy  hedge  orange  (Citrus  trifoliata)). 

In  crosses  between  plants  which  are  too  distantly  related  the 
unfavorable  effect  of  infertility  is  often  partially  obviated  in  culti- 
vation by  propagating  the  offspring  vegetatively.  The  various 
methods  were  discussed  earlier  under  vegetative  reproduction, 
and  need  not  be  repeated  here.  It  is  sufficient  to  repeat  that  by 
budding,  grafting,  and  propagation  by  means  of  bulbs,  tubers, 
and  runners  favorable  varieties  obtained  by  hybridization  are 
now  perpetuated  for  long  periods  of  time.  Hybridization,  there- 
fore, is  one  of  the  most  potent  of  the  methods  available  to  the 
modern  breeder  for  the  production  of  new  kinds  of  ornamental 
and  useful  plants. 


184 


GENERAL  BOTANY 


MENDEL'S  PRINCIPLES  OF  HEREDITY 

It  will  be  evident  to  the  student  that  in  the  crossing  of  plants 
for  increased  vigor  and  for  new  combinations  of  characters  there 
is  a  large  element  of  chance  in  the  methods  of  Darwin,  Burbank, 
and  other  plant  breeders.  The  results  attained  are  indeed  re- 
markable and  are  of 
the  greatest  value 
in  the  improvement 
of  plants  for  man's 
use,  but  they  lack 
the  certainty  and  the 
definiteness  of  a  sci- 
entific method.  The 
breeder  of  plants 
needs  to  be  in  the 
position  of  the  chem- 
ist with  reference 
to  the  characters  of 
the  plants  which  he 
wishes  to  combine 
by  crossing.  The 
chemist  can  com- 
bine the  atoms  and 
molecules  of  two 


Grandchildren,  F2 
"Segregation  "  of  SS 

and  ww  and 
"  mathematical  ratio  " 


FIG.  96.    Diagram   illustrating  Mendel's  results  in 
crosses  between  smooth  and  wrinkled  peas 


Great-grandchildren,  F£ 
Purity  of  SS  and  w  w 
demonstrated 

substances  in  a 
chemical  flask,  and 
by  reason  of  known 
laws  of  chemistry 
he  can  predict  with 
certainty  the  new  compound  or  compounds  that  will  result.  In 
a  similiar  manner  the  breeder  of  plants  needs  to  know  both  the 
nature  of  the  characters  of  the  plants  with  which  he  deals  and 
the  laws  by  which  these  characters  will  combine  in  the  zygote 
to  form  the  kind  of  offspring  which  will  meet  his  need  or  his 
desire.  Mendel's  principles  of  heredity  now  supply  something 
like  this  scientific  basis  for  plant  breeding  and  improvement. 


PLANT  BREEDING  AND  EVOLUTION  185 

Gregor  Mendel  was  an  Austrian  monk  who  in  1865  began 
experiments  on  heredity  in  the  gardens  connected  with  the  mon- 
astery at  Briinn,  Austria.  He  experimented  on  peas,  and  in  his 
studies  he  selected  only  certain  easily  recognizable  characters  for 
observation  in  his  experimental  plants.  Some  of  these  characters 
related  to  the  seeds,  such  as  seed  color,  the  smobth  or  wrinkled 
character  of  the  seed  coat,  and  the  sizes  of  seeds;  tallness  and 
shortness  of  stems  and  hairiness  and  smoothness  in  leaves  were 
other  characters  relating  to  the  plant  body  which  Mendel  selected 
for  observation. 

Mendel 's  method  of  observing  a  few  characters  only,  instead 
of  attempting  to  observe  all  the  characters  in  the  plants  under 
observation,  is  the  most  distinctive  feature  of  his  work.  It 
marked  a  new  epoch  in  the  study  of  heredity,  the  importance 
of  which  we  can  hardly  overestimate.  When  he  crossed  two 
parents  which  possessed  such  definite  contrasting  characters 
as  tall  and  short  stems,  or  smooth  and  wrinkled  seeds,  Mendel 
found  that  the  appearance  of  these  characters  in  the  offspring 
followed  definite  laws,  or  principles.  These  laws,  or  princi- 
ples, which  are  now  called  Mendel's  principles  of  heredity, 
may  be  illustrated  by  giving  Mendel's  results,  obtained  in  an 
experiment  with  seed  characters  in  peas.  Mendel  chose  peas 
because  they  are  self-pollinating,  so  that  it  was  unnecessary 
to  pollinate  the  stigmas  artificially  where  self-pollination  was 
desired.  In  the  experiment  illustrated  in  Fig.  96  he  chose 
two  pure  strains  of  peas,  which  he  had  previously  tested  as  to 
their  purity.  In  one  of  these  strains,  used  as  either  a  male  or  a 
female  parent  in  the  experiment,  the  seeds  were  all  smooth  and 
round  ($$),  while  in  the  other  parent  the  seeds  were  all  wrinkled 
(ww).  When  plants  grown  from  these  parent  seeds  were  cross- 
pollinated  artificially,  the  seeds  thus  produced  were  all  smooth 
like  one  parent.  Since,  however,  these  hybrid  children  (Sw)  had 
received  both  the  smooth  and  the  wrinkled  character  from  the 
two  parents,  Mendel  concluded  that  the  character  in  the  germ 
which  determined  the  smoothness  of  the  seeds  was  dominant  over 
the  wrinkling  character.  He  therefore  called  the  smooth  char- 
acter dominant  and  the  wrinkling  character  latent  or  recessive. 


186 


GENERAL  BOTANY 


Dominance  is  indicated  in  the  figure  by  the  large  size  of  the 
letter  S,  as  compared  with  the  small  size  of  the  symbol  w,  used 
for  the  wrinkling  character. 

The  Mendelian  ratio.  Mendel's  prediction  that  the  characters  for 
both  smooth  and  wrinkled  seeds  existed  in  the  hybrid  children 
($w)  was  prove'd  to  be  correct  by  his  next  experiment.  lie  allowed 
the  plants  produced  by  the  hybrid  children  (Sw)  to  close-pollinate 

and  produce  soeds, 
the  grandchildren  of 
Fig.  90.  By  observing 
and  counting  a  large 
number  of  these  seeds 
he  found  that  the  total 
number  of  smooth 
seeds  bore  the  same 
ratio  to  the  total  num- 
ber of  wrinkled  seeds 
as  that  represented 
in  the  single  pod  in 
the  figure  ;  namely,  as 
3  to  1.  This  is  now 
known  as  the  Men- 
delian ratio  for  the 
hybrids  of  the  first 
filial  generation  (7^, 
children)  resulting  from  a  cross  between  parents  that  have 
distinctive  characters,  as  in  peas  and  Mirabilis  (Fig.  97). 

Furthermore,  he  found  that  plants  grown  from  the  ww  seeds 
always  bore  pure  ww  seeds,  represented  as  great-grandchildren, 
indicating  that  the  original  parental  recessive  strain  had  reap- 
peared in  the  second  hybrid  generation  (2^,  grandchildren)  in 
its  original  purity. 

When,  however,  the  smooth  seeds  were  planted,  the  plants 
thus  produced  were  found  to  contain  some  pure,  smooth-seeded 
forms  ($$),  like  the  original  dominant  parent,  and  some  hybrid 
forms  ($w,  Sw).  The  grandchildren,  therefore,  when  judged  by 
the  seeds  (Fig.  96),  contained  two  strains  ($$  and  ww)  which 


FIG.  97.  Diagram  illustrating  crossing  in  Mirabilis 

The  cross  between  the  white-flowered  and  red-flowered 

Mirabilis  jalapa  (four-o'clock)  gives  an  intermediate 

pink  in  F\.    The  usual  Mendelian  ratio  is  shown  in 

F2.    After  Morgan 


PLANT  BREEDING  AND  EVOLUTION  1ST 

exactly  resembled  the  original  parents  (8S  and  ww).  These 
two  strains,  when  inbred,  proved  to  be  pure  strains,  as  indicated 
by  the  great-grandchildren  (SS  and  ww).  The  grandchildren 
also  carried  a  hybrid  strain  (Sw),  as  indicated  above,  which  by 
inbreeding  split  up  into  pure  strains  (SS  and  ww)  and  into  a 
hybrid  strain  (Sw). 

This  is  clearly  shown  in  the  great-grandchildren  offspring  of 
$w  in  the  figure.  Moreover,  these  pure  and  hybrid  grandchildren 
occurred  in  a  definite  mathematical  ratio  of  |-  pure  SS,  J  pure  ww, 
and  J-  hybrid  Sw.  From  this  it  follows  that,  on  the  average, 
out  of  every  four  grandchildren  one  would  be  pure  SS,  one 
would  be  pure  ww,  and  two  would  be  hybrid  Sw.  This  ratio, 
established  by  Mendel  in  experiments  with  peas,  has  been  found 
to  hold  true  for  a  large  number  of  plant  and  animal  characters. 
In  the  case  of  other  characters  there  is  still  much  doubt  concern- 
ing the  applicability  of  Mendel's  ratio.  The  fact  remains,  how- 
ever, that  Mendel's  method  of  working  with  single  characters, 
and  his  demonstration  of  a  possible  law  for  the  combination  of 
these  characters  in  the  offspring,  has  proved  to  be  of  immense 
theoretical  and  practical  value  to  experimenters  in  heredity  and 
plant  breeding.  These  practical  applications  of  Mendel's  theories 
will  be  discussed  at  the  end  of  the  chapter. 

Gametic  purity.  Mendel's  interpretation  of  such  results  of 
crossing  as  we  have  just  described  will  be  made  clear  by  Fig.  98 
and  the  following  brief  explanation.  Mendel  assumed  that  the 
original  parents  (SS  and  ww)  produced  male  and  female  gametes, 
each  of  which  carried  but  one  of  the  characters  S  and  w.  In  other 
words,  the  two  characters  which  united  at  the  time  of  fertili- 
zation, and  which  appeared  to  occur  together  in  the  seeds  and 
adult  plants,  always  separated  in  the  gametes,  so  that  the 
gametes  (1)  were  pure  and  not  mixed  in  composition,  so  far  as  the 
two  characters  were  concerned.  This  is  known  as  the  law  of 
gametic  purity,  which  is  perhaps  the  most  important  of  Mendel's 
theoretical  conclusions. 

Since,  moreover,  the  male  gametes  are  borne  by  the  pollen 
grains  and  the  female  gametes  are  formed  in  the  embryo  sacs 
of  the  ovules,  chance  pollination  of  the  stigmas  of  a  series  of 


188 


GENERAL  BOTANY 


Gametes  (1),  pure 
S  or  w 


Children, 
ation 

Gametes 
S  or 


gener- 


#),  pure 


flowers  would  be  almost  certain  to  bring  about  all  possible 
combinations  of  S  and  w,  when  fertilization,  or  union,  of  the 
gametes  occurred  to  produce  the  Fl  children. 

Mendel  assumed  also  that  when  the   hybrid  children  (Sw) 

formed  pollen  and  em- 
bryo sacs,   the   deter- 
Parents  miners   for   S   and   w 

would  again  segregate 
and  occur  singly  in 
each  germ  cell.  The 
gametes  (2)  would 
therefore  carry  S  and 
w  in  the  manner  in- 
dicated in  Fig.  98. 
Chance  pollination  and 
fertilization  would  then 
result  in  the  combina- 
tions of  S  and  w  shown 
in  the  (JQ  grandchil- 
dren. Gametes  (3), 
formed  by  these  grand- 
children might  then 
combine  to  produce 
pure  and  hybrid  off- 
spring in  the  ratio 
shown  in  the  F^  great- 
grandchildren. If,  there- 
fore, we  are  willing  to 
accept  Mendel's  idea 
of  the  purity  of  the 
gametes,  we  are  pre- 
pared to  understand  how  the  offspring  of  hybrids  may  occur 
in  mathematical  ratios  similar  to  those  obtained  by  Mendel. 

Paired  contrasting  characters.  Mendel  also  crossed  peas  in 
which  the  parents  differed  in  two  pairs  of  contrasted  characters. 
In  Fig.  99  the  results  of  such  an  experiment  are  indicated  by 
drawings  and  letters  similar  to  those  employed  in  Fig.  98. 


Grandchildren,  F2 
generation 

Gametes  (3),  pure 
S  or  w 


Great-grandchildren, 
Fs  generation 


FIG.  98.    Diagram   illustrating  Mendel's  law  of 
purity  of  gametes 


PLANT  BKEEDING  AND  EVOLUTION 


189 


In  this  instance,  however,  Mendel  crossed  peas  which  bore 
smooth  yellow  seeds  (£F)  with  peas  having  wrinkled  green 
seeds  (wg)*  The 
large  letters  (SY} 

PARENTS 

(Fig.  99)  show  that 
these  characters  were 
dominant  in  the  off- 
spring, wherever  they 
occurred,  and  the 
small  letters  (wg)  in  GRAND_ 
a  similar  manner  in-  CHILD*EN  F2 
dicate  recessive  char- 
acters. The  children 
of  this  cross  were  hy- 
brids in  (  which  the 
two  pairs  of  charac- 
ters brought  in  by 
the  parents  (£F  and 
wg)  were  supposed 
to  be  mingled  as  in- 
dicated in  the  figure. 
These  children  were 
all  smooth  and  yellow, 
like  one  parent,  on 
account  of  the  domi- 
nance of  S  and  Y 
over  w  and  g.  When 
close-pollinated  or  in- 
bred, these  children 
produced  grandchil- 
dren of  four  different 
classes :  smooth  yel- 
low ($F),  smooth 

green  ($#),  wrinkled  yellow  (wF),  and  wrinkled  green  (wg).  It 
will  be  noted  that  two  of  these  ($F  and  wg)  resemble  exactly 
the  original  parents,  while  two  {Sg  and  wY)  are  new  kinds  of 
offspring  due  apparently  to  a  new  combination  of  the  original 


FIG.  99.    Diagram  illustrating  the  results  derived 

from  crossing  peas  with  two  pairs  of  contrasting 

characters 

SY  (smooth  yellow)  are  dominant  over  wg  (wrinkled 
green) .  The  figure  shows  the  proportionate  number  of 
individuals  in  each  class  of  the  grandchildren  (F2)  and 
also  the  composition  of  each  class  in  terms  of  SYwg. 


190  GENERAL  BOTANY 

parental  characters  ($,  F,  w,  #).  If  we  accept  Mendel's  conclu- 
sions as  to  the  stability  of  plant  characters  and  their  separation 
in  the  germ  cells,  we  can  understand  the  above  four  classes  of 
grandchildren.  In  Fig.  100  the  various  possible  combinations 
of  the  characters  $,  F,  w,  g,  as  they  might  occur  in  the  male 
and  female  gametes  of  the  children  (Fig.  99),  are  shown  above 
and  at  the  left  of  the  large  square.  In  the  small  squares  the 
possible  combinations  of  these  characters  in  the  zygotes  (grand- 
children) are  illustrated ;  assuming  that  wherever  S  or  Y  occur 
they  would  dominate  w  and  g,  the  four  classes  of  grandchildren 
would  appear  in  the  ratio  indicated  by  the  small  circles,  repre- 
senting peas,  in  the  small  squares.  These  ratios  are  shown  in 
Figs.  99  and  100  to  be  9  SY  to  3  Sg  to  3  wY  to  1  wg.  According 
to  the  above  experiments  the  characters  of  plants  seem  to  be 
represented  by  factors  in  the  germ  cells  which  are  capable  of 
reproducing  parental  characters  in  the  offspring  resulting  from 
a  cross.  When  plants  are  crossed,  these  factors  are  reshuffled 
and  appear  in  different  combinations  in  the  offspring.  It  has 
already  been  shown  by  experimental  breeders  that  the  great 
variety  manifested  by  plants  and  animals,  as  regards  color,  form, 
and  structure,  is  due  in  many  instances  to  these  chance  Mende- 
lian  combinations  in  hybrids.  It  is  not  possible  at  the  present 
time  to  state  how  universally  Mendel's  principles  will  apply  to 
the  behavior  of  plant  and  animal  characters  in  hybrids  and  their 
offspring.  Enough  has  been  done,  however,  to  indicate  the  great 
value  of  his  results  in  placing  animal  and  plant  breeding  upon 
a  more  accurate  scientific  basis. 

Practical  applications.  The  results  of  Mendel's  work  which  are 
most. important  to  the  plant  breeder  are  as  follows:  First,  the 
breeder  may  find  out  by  observation  and  experiment  the  various 
desirable  and  undesirable  characters  of  plants  which  he  wishes 
to  cross.  Second,  he  is  certain  that  the  desirable  characters  will 
not  be  contaminated  or  lost  in  crossing,  although  they  may 
appear  to  be  so  in  the  children  of  the  first  parents.  Third,  he 
knows  also  that  by  proper  care  in  self -fertilizing  children  of  the 
first  cross  the  characters  of  the  original  parents  will  reappear  in 
all  possible  combinations  in  the  grandchildren  and  their  progeny 


PLANT  BREEDING  AND  EVOLUTION 


191 


SY 


Sg 


wY 


EGGS 
Sg  wY 


wg 


if  properly  bred  and  cared  for.  When  the  combinations  which  he 
desires  appear,  as  in  Burbank's  Shasta  daisy  and  hybrid  walnut, 
he  can  proceed  to  preserve  the  favorable  combination  which  he 
knows  is  composed  of  stable  characters.  A  favorite  method, 
already  indicated,  of  preserving  such  desirable  combinations  of 
characters  is  by  grafting  and  budding  the  new  variety  on  hardy 
but  otherwise  less  valuable  stock.  Since  plants  grown  by  asexual 
means  always  come  true  to  the 
original  parents,  a  new  com- 
bination of  characters  can  be 
preserved  indefinitely  by  these 
means.  In  other  instances  the 
new  combinations  may  prove 
to  breed  true  by  seeds  when 
the  germ  cells  were  pure  for 
the  characters  combined  in  the 
new  type.  The  type  may  also 
be  maintained  by  careful  con- 
tinued selection.  The  scientific 
breeder  is  thus  able  to  forecast 
results  and  plan  his  crossing  in 
accordance  with  definite  laws. 
In  the  above  brief  survey  of 


FIG.  100.   Diagram  showing  method  of 

determining  the  composition  of  zygotes 

shown  in  Eig.  99 


The  four  classes  of  female  gametes  above 
and  of  male  gametes  at  the  left.  The  pos- 
sible combinations  of  the  gamete  factors 
shown  in  squares.  Considering  S  and  V 
dominant  wherever  they  occur,  we  get  the 
ratio  9  SY  to  3  Sg  to  3  wY  to  1  wg 


Mendel's  work  all  details  and 
exceptions  have  been  neces- 
sarily omitted,  but  enough  has 

been  given  to  indicate  to  the  student  that,  in  the  future,  plant 
breeding  is  likely  to  become  a  science  and  not  a  game  of  chance, 
with  results  of  profound  importance  to  the  improvement  of  plants 
and  animals  for  man's  uses.  If  this  prediction  proves  to  be  cor- 
rect, Mendel  will  always  be  known  as  the  founder  of  a  scientific 
method  in  plant  and  animal  breeding.  It  is  more  than  probable 
also  that  nature  repeats,  in  the  wild,  the  same  experiments  which 
man  has  learned  to  do  in  his  culture  beds  and  experiment 
plots.  Nearly  related  species  in  nature  are  known  to  hybridize, 
and,  such  being  the  case,  the  hybrids  must  combine  the  char- 
acters of  the  parents  in  new  mixtures,  and  then  these  mixtures 


192  GENEKAL  BOTANY 

must  continually  segregate  in  accordance  with  Mendel's  laws 
in  future  generations.  Much  of  the  great  and  pleasing  variety 
in  nature  would  thus  be  produced,  so  that  many  of  the  so-called 
varieties  and  species  of  wild  plants  have  doubtless  arisen  by 
these  multifarious  combinations  and  segregations  of  which  we 
have  had  a  glimpse  in  the  above  sketch. 

PLANT  IMPROVEMENT  BY  SELECTION 
VARIATION  AND  SELECTION 

We  have  just  learned  how  Burbank  and  other  plant  breeders 
have  been  able  to  produce  variations,  or  differences,  in  plants 

by  crossing  and  hy- 
bridizing, owing  to 
the  fact  that  new 
combinations  of  par- 
ental characters  are 
wont  to  appear  in  the 
FIG.  101.  Variation  in  apples  offspring  of  a  cross. 

Four  varieties  of  the  Early  Williams  apple,  selected  Qut  of  these  differ- 
from  thirty-six  varieties  borne  on  seedlings.  Modified  . 

from  photograph  by  Burbank  ing  offspring  the  CUl- 

tivator  selects  those 

plants  which  best  suit  his  needs  or  his  desires,  and  endeavors 
to  perpetuate  their  kind  by  vegetative  means  or  by  collecting 
and  sowing  seed. 

This  is  the  essential  principle  in  all  plant  improvement  by 
the  selection  of  variations,  and  the  general  method  is  familiar  to 
everyone.  It  is  not  always  necessary,  however,  to  resort  to  the 
tedious  process  of  artificially  crossing  plants  in  order  to  secure 
the  desired  variations  with  which  to  start  a  new,  improved  race, 
since  all  plants  normally  produce  offspring  which  vary  among 
themselves  in  almost  every  conceivable  manner.  In  a  field  of 
corn,  for  instance,  some  plants  are  tall  and  others  short ;  in 
some  the  ears  are  produced  well  up  on  the  stalk,  while  others 
are  borne  nearer  the  ground.  The  ears  likewise  vary  in  length, 
in  the  number  of  rows  of  kernels  on  the  ear,  and  in  the  nature 
of  the  stored  food  in  the  kernel.  Some  of  these  differences  in 


PLANT  BREEDING  AND  EVOLUTION 


193 


Tassel 


corn  are  due  to  natural  crossing  in  the  field,  since  corn  is  nor- 
mally cross-pollinating.  There  is  evidence  also  that  every  field 
of  corn  of  the  same  kind  or  va- 
riety is  made  up  of  minor  races 
(elementary  species),  which  differ 
among  themselves  in  some  or 
all  of  the  characters  mentioned 
above.  These  races  cross  and 
produce  the  same  kind  of  varia- 
tions as  arise  from  artificial  cross- 
ing and  hybridizing.  In  other 
instances  the  variations  are  due 
to  differences  in  soil,  water  sup- 
ply, climate,  and  tillage.  Out  of 
these  multitudinous  differences 
the  corn  breeder  selects  the  plant 
or  plants  which  suit  his  ideal, 
and  proceeds  to  secure  an  im- 
proved race  by  careful  selection 
and  cultivation.  Fig.  103  illus- 
trates the  great  variability  in  ears 
of  corn  in  the  same  field,  due  to 
the  indiscriminate  crossings  be- 
tween plants  bearing  poor  and 
good  ears.  Fig.  104  illustrates 
the  great  improvement  in  uni- 
formity and  yield  which  can  be 
secured  by  selecting  the  best  ears 
for  seed  and  by  preventing  cross- 
pollination  with  inferior  types  for 
a  series  of  years. 

The  gradual  increase  in  yield 
of  corn  in  Illinois,  Iowa,  and 
other  great  corn-producing  states 
during  the  past  decade  is  due  in  large  measure  to  the  fact  that 
corn  breeders  have  exercised  more  care  than  they  formerly  did 
in  studying  variations  in  corn  and  in  selecting  proper  seed  for 


Secondary 

roots  Primary  root 

FIG.  102.    A  plant  of  Indian  corn 

Showing  male  inflorescence  (tassel)  and 
ears  ready  for  pollination 


194 


GENERAL  BOTANY 


planting.    It  has  also  been  found  that  not  only  can  the  size  and 
shape  of  the  ear  be  thus  improved  by  selection  but  also  the 

contents  of  the 
kernel,  including 
the  valuable  com- 
mercial products 
derived  from  corn, 
such  as  starch, 
oil,  and  protein. 
In  the  Illinois 
agricultural  ex- 
periment station 
work  was  started 


•"  Showing  great  variability  in  type  resulting  from  failure 

to  select  seed."  Photograph  furnished  by  the  United  States 

Department  of  Agriculture 


FIG.  103.    Boon  County  White  Corn,  unselected 

in  1896   to  pro- 
duce   strains    of 
corn  with  differ- 
ent amounts  of  starch,  fat,  or  protein  as  reserve  food  in  the 
kernels.    The  starch  in  ordinary  varieties  of  corn  forms  the  bulk 
of  the  endosperm,  the  oil  is  located  largely  in  the  embryo,  and 


FIG.  104.   Boon  County  White  Corn,  selected 

Showing  uniformity  in  type  due  to  thirty-two  years  of  seed  selection.   Photograph 
furnished  by  the  United  States  Department  of  Agriculture 

the  protein  is  found  in  the  outer  portion  of  the  endosperm  next 
to  the  seed  coat.  By  sectioning  kernels  on  the  ears  of  different 
plants  the  amount  of  starch,  fat,  or  protein  can  be  judged  by 


PLANT  BREEDING  AND  EVOLUTION 


195 


the  amount  of  starchy  endosperm,  the  width  of  the  protein  layer, 
or  the  size  of  the  embryo  (Figs.  105  and  106).    In  this  manner 

"  one  hundred  sixty-three 
ears  of  Burr  White  Maize 
from  the  1896  crop  were 
analyzed  and  the  proper 
selections  in  each  case 
planted  in  their  respective 
plots.  In  each  plot  from 
twelve  to  twenty-four 
rows  were  planted,  and 
each  row  contained  only 
the  kernels  from  a  single 
ear.  In  succeeding  years 
a  hundred  or  more  ears 
were  analyzed  from  each 
breeding  plot,  and  those 
ears  from  the  high-protein 
plot  which  showed  the 
highest  per  cent  of  protein 
were  saved  to  plant  the 
next  year's  high-protein 
plot,  while  those  from  the  low-protein  plot  were  saved  to  plant  the 
low-protein  plot  of  the  next  season.  Like  methods  were  followed 
in  the  high-oil  and  low- 
oil  breeding  plots."  -  •?-  - 

By  this  method  the  oil, 
which  is  called  corn  oil, 
was  increased  in  six  years 
in  a  strain  of  corn  from 
4.7  per  cent  to  nearly 
7  per  cent,  while  the 
protein  used  in  making 
hominy  grits  and  for 
feeding  stock  was  increased  from  10.6  per  cent  to  16  per  cent. 
The  same  general  procedure  outlined  above  for  the  produc- 
tion of  improved  types  of  corn  by  the  selection  of  favorable 


FIG.  1C5.    Kernels  of  corn  with  high  and 
low  percentage  of  oil 

A,  AI,  transverse  section  and  face  view  of  ker- 
nels with  large  embryo  having  high  oil  content ; 

B,  />!,  same  views  of  kernels  with  small  embryos 
having  low  oil  content.    From  Bulletin  87,  Uni- 
versity of  Illinois  Agricultural  Experiment  Station 


A 

FIG.  106.    Kernels  of  corn  with  high 
percentage  of  protein 

A,  high-protein  kernel;   />,   low-protein  kernel; 

e,  embryo;  s,  starchy  area;  p,  horny,  or  protein, 

layer.-    From  Bulletin  87,  University  of  Illinois 

Agricultural  Experiment  Station 


196  GENERAL  BOTANY 

variations  is  followed  in  the  improvement  of  other  kinds  of  fruit, 
forage,  and  food-producing  plants.  The  following  extract  from 
"  Plant  Breeding,"  by  Professor  Bailey,  illustrates  some  of  the 
detail  and  care  necessary  in  such  a  selecting  process  when  applied 
to  the  cultivation  of  beans. 

He  [the  breeder]  starts  with  one  plant.  The  next  year  he  may 
have  only  two.  If  he  has  ten  or  twenty  good  ones,  then  the  task  is 
easy,  for  the  variety  has  elements  of  permanence,  that  is,  of  heredi- 
tability,  in  it.  As  soon  as  seeds  can  be  secured  in  considerable 
amount  from  a  strain  of  beans,  selected  as  indicated  above,  the 
grower  can  plant  a  large  plot  and  obtain  seeds  for  sale  or  for  dis- 
tribution to  other  breeders.  He  must  exercise  judgment  and  skill 
each  year,  however,  in  selecting  seed,  even  from  such  a  carefully 
improved  race,  if  he  is  to  prevent  its  running  out  or  reverting  to 
the  original  type.  This  is  due  to  the  fact  that  his  fields  and  experi- 
mental plots  will  always  contain  reversions,  or  "  rogues,"  from  which 
seed  must  not  be  taken  if  his  improved  strain  is  to  be  kept  true  to 
type.  In  the  common  dwarf,  or  bush,  beans  of  the  gardens,  for 
instance,  there  is  always  a  tendency  to  revert  to  the  ancestral  climb- 
ing variety.  These  rogues  with  a  tendency  to  climb  must,  there- 
fore, be  eliminated  if  the  bush  habit  is  to  be  perpetuated. 

Mr.  Palmer's  dwarf  lima  originated  in  1883,  when  his  entire  crop 
of  large  white  (pole)  limas  was  destroyed  by  cutworms.  He  went 
over  his  field  to  remove  the  poles  before  fitting  the  land  for  other 
uses,  but  he  found  one  little  plant,  about  ten  inches  high,  which  had 
been  cut  off  about  an  inch  above  the  ground,  but  which  had  rerooted. 
It  bore  three  pods,  each  containing  one  seed.  These  three  seeds 
were  planted  in  1884,  and  two  of  the  plants  were  dwarf  like  the 
parent.  By  discarding  all  plants  which  had  a  tendency  to  climb, 
in  succeeding  crops,  the  Burpee  bush  lima,  as  we  now  have  it, 
was  developed. 

Difficulties.  Two  main  difficulties  are  experienced  by  breeders 
in  their  attempt  to  improve  plants  by  the  selection  of  variations. 
One  of  these  difficulties  has  already  been  referred  to,  namely, 
the  tendency  of  varieties  and  races  thus  produced  to  revert  to 
the  condition  of  ancestors  of  the  less  desirable  type,  a  phenomenon 
expressed  by  the  term  running  out.  A  second  point  of  impor- 
tance is  that  races  built  up  by  the  continued  selection  and 


PLANT  BREEDING  AND  EVOLUTION 


197 


accumulation  of  variations  are  limited  in  the  degree  to  which 
they  can  be  improved.  Thus,  the  sugar  beet  has  been  cultivated 
and  selected  for  a  hundred  years  or  more,  but  it  has  not  been 
found  possible  to  increase  the  average  sugar  production  of  a 
selected  field  of  beets  beyond  from  16  to  18  per  cent.  The  Illi- 
nois experiment  station  has  had  a  similar  experience  in  attempt- 
ing to  raise  the  percentage  of  oil  and  protein  in  corn  beyond 
the  high  percentages  mentioned  above.  A  similar  condition 
seems  to  prevail  among  all  cultivated  varieties,  where  ordinary 
variations  are  used  as  a  basis  for  selection. 


Two  KINDS  OF  VARIATIONS 

Two  kinds  of  variations  are  now  recognized  by  scientists 
and  plant  breeders,  known  respectively  as  fluctuating  (or  con- 
tinuous) variations,  and  mutations  (or  discontinuous  variations). 
.Fluctuating  variations 
are  the  ordinary  dif- 
ferences observed  in 
plants  of  the  same 
kind  in  a  field  or  a 
garden.  They  are  due 
to  the  environment, 
such  as  rich  or  poor 
soil,  abundance  or 
lack  of  water,  sun  ex- 
posure or  shade,  and 
many  other  environ- 
mental influences  to 
which  plants  growing 
in  the  open  are  sub- 
jected. They  are  fur- 
thermore inconstant, 
not  tending  to  be  perpetuated  in  the  offspring  of  plants  mani- 
festing them.  We  all  know,  for  instance,  that  seeds  from  a 
large,  healthy  tomato  plant,  produced  by  rich  soil  and  careful 
culture  in  a  garden,  will  not  bear  similar  offspring  unless  they 


FIG.  107.    Connecticut  broad-leaf  tobacco, 
unselected 

Showing  variation  in  tobacco  due  to  change  of  soil 

and  climate.    Photograph  furnished  by  Connecticut 

Agricultural  Experiment  Station 


198 


GENEKAL  BOTANY 


grow  under  like  conditions.  It  seems  probable,  therefore,  that 
the  running  out  of  selected  races  of  corn,  beans,  and  other 
cultivated  plants  mentioned  in  the  previous  pages  is  due  to 
the  selection,  on  the  part  of  cultivators  and  breeders,  of  plants 
with  these  fluctuating  variations.  Fluctuating  variations  can  be 
accumulated  to  a  certain  extent,  and  a  better  race  may  thus  be 

produced ;  but  such 
a  race  will  always  re- 
main inconstant  and 
limited  in  range  or  in 
the  degree  to  which 
it  can  be  improved. 
Thus,  in  the  tobacco 
plant  (Fig.  107)  great 
variation  has  been  in- 
duced in  certain  varie- 
ties by  planting  in  the , 
soil  and  climate  of 
Connecticut  and  other 
Northern  states  seed 
obtained  in  Sumatra 
and  Florida.  Some  of 
these  variations  are  of 
the  fluctuating  variety, 


Showing  uniformity  of  type  in  tobacco  due  to  selec- 
tion of  seed.  Photograph  by  the  United  States  Depart- 
ment of  Agriculture 


FIG.  108.    Uncle  Sam  Sumatra  tobacco,  selected 

and  so  will  not  form 
the  basis  for  a  stable 
improved  race  pro- 
duced by  selection.  In  other  instances  the  variations  induced  in 
tobacco  by  the  change  in  climate  and  soil  are  apparently  stable 
from  the  outset,  and  form  the  basis  for  new  constant  races. 
The  Uncle  Sam  Sumatra  tobacco  (Fig.  108)  is  supposed  to  have 
had  such  an  origin  by  mutation. 

The  Florida-Sumatra  tobacco  was  grown  from  seed  secured  from 
the  island  of  Sumatra.  When  the  Florida-grown  seed  was  taken  to 
Connecticut,  the  plants  grown  from  it  varied  in  a  marked  degree,  and 
several  new  types  developed  that  did  not  exist  in  Florida  and,  accord- 
ing to  the  best  information  obtainable,  did  not  exist  in  Sumatra. 


PLANT  BKEEDING  AND  EVOLUTION 


199 


I 


The  Uncle  Sam  type  appeared  among  these  variations  and  has 
proved  to  be  constant  when  grown  from  seed.  In  tobacco,  there- 
fore, as  in  many  other  plants,  two  kinds  of  variations  frequently 
occur  which  are  of  very  different  value  and  importance  in  the 
improvement  of  these  plants  by  breeding.  The  inconstant,  or 
fluctuating,  variations  are  not  of  great  value  for  selection  in 
the  making  of  an  improved 
race,  while  the  stable  varia- 
tions called  mutations  are 
of  the  greatest  interest  and 
importance  to  breeders  of 
new  kinds  of  plants  and 
animals. 


THE  MUTATION  THEORY 

Certain  varieties  of  plants, 
like  the  Uncle  Sam  Sumatra 
tobacco,  have  long  been 
known  to  have  arisen  sud- 
denly as  a  result  of  varia- 
tions affecting  one  or  more 
parts  of  the  plant  body. 
The  plants  manifesting  these 
new  variations  have  usually 
been  called  sports  in  agri- 
culture and  horticulture,  but  are  now  called  mutants.  The 
variations  which  distinguish  them  from  the  parent  species  are 
called  mutations.  Among  the  best  known  of  such  sports,  or 
mutants,  are  the  following:  the  moss  rose  and  the  nectarine, 
which  are  bud  sports  from  a  cultivated  rose  and  from  the 
peach ;  the  various  cut-leaved  varieties  of  the  willow,  maple, 
and  birch ;  many  white-flowered  varieties  springing  from  plants 
with  colored  flowers ;  and  probably  many  varieties  of  vegetables, 
forage  plants,  cereals,  and  fruits  whose  history,  when  traced 
back,  indicates  a  sudden  origin  from  wild  species.  The  muta- 
tion theory  for  the  origin  of  new  varieties  by  sudden  constant 


FIG.  109.    (Enothera  lamarckiana 

This  is  the  mother  plant  of  mutants  dis- 
covered and  produced  by  De  Vries.    From 
Babcock  and  Clausen's  "  Genetics  in  Rela- 
tion to  Agriculture."   After  De  Vries 


200 


GENEKAL  BOTANY 


variations  has  been  brought  into  prominence  in  recent  years 
through  the  publications  of  the  great  Dutch  botanist,  Hugo  de 
Vries.  De  Vries  first  noticed  some  mutants  of  the  evening 
primrose  known  as  Lamarck's  evening  primrose  {(Enothera 
lamarckiana)  (Fig.  109)  growing  in  a  waste  field  near  Hilver- 

sum  in  Holland.  In  1886 
he  collected  seeds  from 
the  mother  plant  and  from 
the  two  new  varieties  pro- 
duced from  it  by  muta- 
tion, and  sowed  them  in 
the  botanic  gardens  at 
Amsterdam.  He  has  since 
carried  on  extensive  cul- 
ture experiments  with  the 
primroses  and  their  off- 
spring, and  has  succeeded 
in  producing  several  new 
varieties  which  differ  from 
the  original  mother  plant 
in  all  of  the  various  or- 
gans of  the  plant  body. 
One  is  a  dwarf  species 
(Fig.  110),  while  another 
is  a  giant  form  with 
greater  vegetative  vigor 
and  larger  flowers  than 
the  mother  plant.  Others 
vary  in  the  form  and 
color  of  the  leaves,  in  the  character  of  the  seedlings  (Fig.  Ill), 
and  in  the  nature  of  the  reproductive  organs,  including  both 
the  flower  and  the  fruit.  Since  all  of  these  new  primroses 
came  suddenly,  by  one  large  variation  or  mutation,  and  have 
bred  true  to  seed,  De  Vries  believes  that  they  indicate  the 
way  in  which  other  cultivated  and  wild  varieties  have  arisen 
whose  history  indicates  a  similar  method  of  production.  This 
sudden  origin  of  cultivated  forms  is  well  illustrated  by  the 


FIG.  110.    CEnothera  lamarckiana  and  two  of 
its  mutants,  (E.  lata  and  (E.  nanella 

(E.  lamarckiana  in  the  middle;  lata  at  the  left; 

and  nanella  at  the  right.    From  Babcock  and 

Clausen's  "  Genetics  in  Relation  to  Agriculture." 

After  De  Vries 


HUGO  DE  VRIES 

Hugo  de  Vries  is  director  of  the  Botanic  Garden  in  Amsterdam,  Holland.  He  is  the 
greatest  living  exponent  of  the  origin  of  species  by  sudden  constant  variations,  or 
mutations.  His  early  investigations  on  variation,  which  convinced  him  of  the  truth  of 
the  mutation  theory,  extended  over  a  period  of  nearly  twenty-six  years  and  involved 
experiments  and  breeding  tests  with  some  hundred  species  of  wild  plants.  His  experi- 
ments and  theories  are  set  forth  in  his  greatest  work,  "Die  Mutations  Theorie,"and 
in  numerous  succeeding  books  and  articles.  In  his  method  of  work  and  in  the  im- 
portance of  his  results  De  Vries  is  perhaps  more  nearly  comparable  to  the  great 
Darwin  than  any  other  living  investigator 


202 


GENERAL  BOTANY 


history   of   certain    species    of   grapes   recounted   by   Professor 
Bailey  in  his  "  Evolution  of  our  Native  Fruits." 

The  Concord  was  a  chance  seedling  in  a  Massachusetts  garden, 
and  it  is  supposed  to  have  sprung  from  the  wild  fox  grape  of  the 

neighborhood. 

The  Clinton  came  up 
where  a  handful  of 
grape  seeds  was  sown 
at  Hamilton  College, 
Clinton,  New  York ; 
and  the  old  vine,  now 
nearly  seventy-five 
years  old,  is  still  grow- 
ing on  College  Hill. 

The  Norton's  Vir- 
ginia was  found  wild 
in  1835  near  Rich- 
mond, Virginia. 

The  best  Ameri- 
can gooseberries,  the 
Hough  ton  and  the 
Downing,  are  sup- 
posed, like  the  Con- 
cord and  Clinton 
grapes,  to  have  origi- 
nated from  chance 
FIG.  111.  Seedlings  of  (E.  lamarckiana  and  three  -,-,.  .  ,  ,, 

of   its  mutants,   showing   constant  differences  in  m£S>  . 

rosettes  and  leaf  characters  Swedish  experiment 

Upper  row,  typical  lamarckiana ;  second  row  from  top,      station,      located     at 

(E.  gigas;  third  row,  (E.  rubrinervis :  bottom  row,  (E.      QUralof  <«s™-f}rlcm    Y\O\\T 

lata.   Photograph  furnished  by  Dr.  George  H.  Shull         ^ValOt'  ^W6(  ^  new 

kinds  of  grains,  for- 
age crops,  and  vegetables  have  been  discovered  in  the  field 
and  garden  which  are  believed  to  have  arisen  by  mutation, 
and  similar  discoveries  have  been  made  in  different  experiment- 
stations  in  this  country.  Many  plant  breeders  and  botanists 
believe,  therefore,  that  the  origin  of  new  forms  of  plants  by 
mutations,  or  sudden  variations,  is  a  widespread  phenomenon 


PLANT  BREEDING  AND  EVOLUTION 


203 


in  nature,  which  bids  fair  to  be  of  inestimable  value  in  the 

improvement  of  wild  and  cultivated  ornamental  and  food  plants. 

Selection  of  mutations.   According  to  the  above  theory  a  field 

of  oats,  corn,  timothy,  or  beans  is  likely  to  be  made  up  of  plants 


FIG.  112.   Inflorescences  and  rosettes  of  CE.  lamarckiana  and  (E.  gigas 

Left,  (E.  lamarckiana ;  right,  (E.  gigas.   Above,  inflorescences ;  below,  young  plants 
(seedlings).  From  Babcock  and  Clausen's  "  Genetics  in  Relation  to  Agriculture  " 

which  differ  from  each  other  in  almost  every  conceivable  manner* 
Some  of  these  differences  would  certainly  be  due  to  fluctuating 
variations,  and  some  to  previous  hybridizations.  In  other  in- 
stances, if  the  experience  of  the  Swedish  and  American  experi- 
ment stations  is  a  criterion  for  judgment,  the  differences  are 
due  to  mutations,  possibly  induced  by  changes  in  soil,  climate, 
or  culture  methods. 


204 


GENERAL  BOTANY 


The  new  forms  thus  produced  by  mutation  are  constant  small 
species,  or  "  elementary  species,"  termed  mutants.  If  these  new 
plants  can  be  recognized  and  selected,  and  the  seeds  sown  in 

isolated  breeding  plots,  it  is  pos- 
sible to  secure  a  new  constant 
strain  at  once  without  further 
selection.  In  reporting  upon  the 
work  of  the  Swedish  station  De 
Vries  says: 

Besides  wheat,  the  cultures  of 
oats,  peas,  and  vetches  are  seen  to 
produce  sports  from  time  to  time  at 
Svalof.  Here  also  the  sports  are 
sudden  and  without  preparation  or 
intermediates,  each  of  them  at  once 
constituting  a  new  type,  which  is 
as  distinct  from  its  allies  as  any 
new  form  found  in  the  fields. 


FIG.  113.    Variation  in  timothy 

The  two  lower  plants  grew  side  by  side  from  the  same  seed.    Desirable  type  at  the 

right ;  dwarf  plant  at  the  left.    Dwarf  plant  with  large  spikes  in  the  upper  figure. 

Photograph  by  New  York  Agricultural  Experiment  Station 


PLANT  BKEEDING  AND  EVOLUTION 


205 


In  this  manner  improved  races  of  cereals  and  forage  crops, 
adapted  to  the  varying  soil  and  climatic  conditions  of  the 
country,  have  been  obtained  in  Sweden  at  the  Svalof  station. 

This  method  of  selecting  seeds  from  one  plant  and  growing 
them  in  isolated  culture  plots  (Fig.  114),  so  that  the  progeny  of 
single  individuals  can  be  observed  and  recorded,  is  called  pedi- 
gree culture.  Pedigree  culture  has  been  practiced  for  many  years 
in  the  breeding  of  animals,  but  its  application  to  plant  breeding 


FIG.  114.    Centgener  plots  of  flax  and  beans  alternating 

One  hundred  seeds  are  planted  in  each  centgener  plot.  Photograph  furnished  by 
the  United  States  Department  of  Agriculture 

has  been  greatly  stimulated  in  the  experiment  stations  of  this 
country  by  the  experiences  of  the  Swedish  station  mentioned 
above.  Pedigree  culture  is  thus  very  different  from  the  older 
mass-culture  method,  by  which  seed  is  collected  from  a  consider- 
able number  of  the  best  plants  in  a  field  and  then  sowed  en 
masse,  without  any  attempt  to  trace  or  record  the  progeny  of 
one  particular  plant.  The  mass-culture  method  is  often  used 
for  selecting  the  better  varieties  from  a  general  field,  and  is  then 
followed  by  pedigree  culture  applied  to  a  few  best  individuals 
selected  from  the  mass-culture  plots. 


206  GENERAL  BOTANY 

SUMMARY 

Variations.  Variations  form  the  materials  from  which  new  and 
improved  races  of  plants  are  built  up,  either  in  wild  nature  or  in 
culture.  These  variations  arise  in  three  ways ;  namely,  by  crossing 
and  hybridization,  by  induced  or  spontaneous  fluctuating  variations, 
and  by  mutation. 

Selection  is  the  method  employed  in  all  cases  for  the  isolation 
of  a  new  race,  or  for  the  gradual  accumulation  of  variations  in  the 
direction  desired  by  the  cultivator. 

Selection  and  hybridization.  The  variations  due  to  hybridizations 
and  to  crossings  usually  arise  from  the  combination  and  reshuffling 
of  characters  which  originally  existed  in  the  parents  of  a  cross.  The 
offspring  produced  by  a  cross  split  in  succeeding  generations,  in 
accordance  with  Mendel's  laws,  into  a  great  variety  of  forms,  in  which 
the  parental  characters  are  differently  combined.  Out  of  the  variety 
thus  produced  new  and  improved  races  can  be  selected  and  multiplied 
by  pedigree  culture. 

Selection  of  fluctuating  variations.  The  improvement  of  plants 
by  the  continuous  selection  of  forms  which  vary  in  the  desired 
direction  is  one  of  the  oldest  methods  of  securing  improved  races 
of  cultivated  plants.  "  Nature  gives  successive  variations ;  man 
adds  them  up  in  certain  directions  useful  to  himself.  .  .  .  The  key  is 
man's  power  of  accumulative  selection."  Darwin  thus  expressed 
the  essential  facts  relating  to  the  improvement  of  plants  by  the 
selection  of  fluctuating  variations.  The  points  of  disadvantage  in 
the  method  are,  first,  that  races  and  varieties  thus  produced  are 
unstable,  tending  to  run  out  when  selection  ceases ;  and,  secondly, 
that  they  are  limited  in  the  degree  to  which  they  can  be  improved 
by  selection. 

Selection  of  mutations  and  pedigree  culture.  Mutations  are  varia- 
tions in  one  or  more  characteristics  of  a  plant  which  arise  suddenly 
and  are  stable  from  the  beginning.  These  mutations  are  either 
small,  creating  the  impression  of  a  mere  variation  in  leaf,  flower,  or 
fruit,  or  they  may  be  large  enough  to  produce  at  once  a  new  kind 
of  plant,  which  will  be  recognized  by  botanists  as  a  new  variety  or 
small  species,  now  called  an  elementary  species.  The  recognition 
and  selection  of  mutations,  especially  those  large  enough  to  consti- 
tute a  new  elementary  species,  is  one  of  the  surest  and  best  methods 
for  securing  an  improvement  in  any  variety  of  plant.  The  reason 


CHARLES  DARWIN 

Charles  Darwin  published  his  first  great  book,  "  On  the  Origin  of  Species,"  in  1859. 
This  book  was  the  outcome  of  over  twenty  years'  work  spent  in  collecting  data  on 
problems  relating  to  variation,  heredity,  competition,  and  survival  in  organisms. 
Darwin's  later  publications  and  investigations  were  designed  to  vindicate  and  extend 
the  theories  and  facts  contained  in  the  "  Origin  of  Species."  It  is  safe  to  say  that  no 
other  scientific  man  ever  exercised  a  greater  influence  on  the  thought  and  progress 
of  mankind.  From  Bergen  and  CaldwelFs  "  Introduction  to  Botany  " 


208  GENERAL  BOTANY 

for  this  is  that  the  forms  thus  selected  remain  true  in  cases  where 
the  new  race  is  produced  by  hybridization.  The  detection  and 
cultivation,  by  pedigree-culture  methods,  of  mutations  occurring 
in  field  and  garden  crops  is.  one  of  the  most  modern  phases  of  plant 
improvement  and  is  an  important  addition  to  the  older  methods 
of  mass  culture  and  hybridization. 

EVOLUTION 

The  facts  and  processes  relating  to  the  evolution  of  plants 
cannot  be  dealt  with  at  any  considerable  length  in  an  elementary 
textbook.  The  beginning  student  should  understand,  however, 
the  close  relation  which  exists  between  the  methods  used  by  man 
in  the  improvement  of  plants  outlined  in  the  preceding  pages  and 
the  process  of  evolution  among  wild  species  in  nature.  Indeed, 
Charles  Darwin,  the  greatest  of  all  students  and  writers  on  evolu- 
tion, took  the  experience  arid  results  of  plant  and  animal  breeders 
as  the  basis  for  his  theories  and  investigations  in  evolution. 

Selection  of  fluctuating  variations.  Darwin  thought  that  new 
species  arose  in  wild  forms  of  life  by  the  gradual  accumulation  of 
fluctuating  variations,  in  much  the  same  way  as  a  plant  breeder 
now  produces  a  larger  fruit  or  a  more  beautiful  flower  by  con- 
stantly selecting  the  best  plants  from  those  which  tend  to  vary 
in  a  given  direction.  Darwin  also  endeavored  to  show  that  a 
process  of  selection  takes  place  in  wild  nature  not  unlike  that 
exercised  by  man,  due  to  the  competition  and  struggle  occurring 
among  plants,  caused  by  the  overproduction  of  seeds.  "  Can  it 
then  be  thought  improbable,"  says  Darwin,  "  seeing  that  varia- 
tions useful  to  man  have  occurred,  that  other  variations,  useful 
in  some  way  to  each  being  in  the  great  and  complex  battle  of 
life,  should  occur  in  the  course  of  many  successive  generations. 
If  such  do  occur,  can  we  doubt  (remembering  that  many  more 
individuals  are  born  than  can  possibly  survive),  that  individuals 
having  any  advantage,  however  slight,  over  others,  would  have 
the  best  chance  of  surviving  and  procreating  their  kind.  This 
preservation  of  favorable  individual  differences  and  variations, 
and  the  destruction  of  those  which  are  injurious,  I  have  called 
natural  selection,  or  the  survival  of  the  fittest " 


PLANT  BREEDING  AND  EVOLUTION  209 

The  ideas  concerning  selection  through  struggle  for  existence, 
emphasized  in  the  above  extract  from  the  "  Origin  of  Species," 
can  be  most  easily  understood  from  concrete  illustrations.  The 
writer  once  made  an  estimate  of  the  number  of  seeds  borne  by 
the  common  dandelion  in  a  single  season.  Rather  large,  healthy 
plants  produced  on  the  average  about  ten  fruiting  scapes,  and 
each  seed  ball  on  a  scape  averaged  from  160  to  260  seeds.  This 
would  mean  an  average  of  from  1600  to  2600  seeds  borne  on 
each  dandelion  plant  in  a  single  season.  The  Russian  thistle  has 
been  estimated  to  produce  from  20,000  to  200,000  seeds  .011  a 
single  plant.  If  all  of  these  seeds  gained  a  foothold  and  grew 
into  offspring,  it  can  readily  be  understood  that  the  earth  might 
be  entirely  populated  with  dandelions  or  Russian  thistles  in  a 
comparatively  few  years,  since  both  plants  are  well  adapted  for 
distributing  their  seeds.  It  must  be  remembered,  however,  that 
the  plants  with  which  the  dandelion  and  Russian  thistle  have  to 
compete  for  soil  space,  air,  and  light  are  also  producing  seeds 
and  offspring  at  corresponding  rates,  and  that  some  of  their  com- 
petitors, like  the  grasses,  have  vegetative  means  for  spreading 
which  are  quite  as  effective  as  seed  production.  It  is  conceivable, 
therefore,  as  Darwin  has  said,  that  in  the  fierce  competition 
among  plants,  due  to  overproduction,  only  those  individuals 
would  survive  in  a  given  environment  which  possessed  structural 
and  physiological  characteristics  best  adapted  to  the  conditions 
of  that  environment.  The  remainder  would  die,  and  the  best- 
adapted  plants  would  live  to  perpetuate  the  race,  just  as  the 
particular  races  of  cultivated  plants  now  in  existence  are  sur- 
vivors from  plants  formerly  selected  by  man.  In  this  manner 
any  given  race  would  gradually  improve,  and  so  evolution  would 
take  place.  This  selective  principle  is  also  easily  understood  if 
we  contemplate  extreme  conditions  such  as  obtain  in  deserts  and 
in  lakes  or  ponds.  The  vegetation  of  these  two  habitats  is  very 
distinct,  for  the  reason  that  extreme  drought,  on  the  one  hand, 
and  excessive  water  supply,  on  the  other,  destroy  all  seeds,  seed- 
lings, or  mature  plants  which  are  not  definitely  adapted  to  a 
xerophytic  or  a  hydrophytic  habitat.  Each  year  seeds  from  other 
habitats  are  distributed  over  deserts  and  lakes,  but  only  those 


210  GENERAL  BOTANY 

best  adapted  to  the  conditions  survive,  so  that  the  cacti,  the  bul- 
rushes, and  the  algse  are  in  a  true  sense  the  fittest  survivors, 
selected  by  the  extreme  conditions  of  their  environment. 

These  habitats  are  not  unlike  a  very  exacting  plant  breeder, 
who  ruthlessly  discards  all  plants  in  his  culture  plots  which  do 
not  conform  to  a  rigid  ideal  which  he  has  started  out  to  attain. 
In  the  development  of  cultivated  races  from  fluctuating  varia- 
tions it  has  been  found,  however,  as  has  already  been  stated,  that 
such  races  are  both  unstable  and  limited  in  the  degree  to  which 
a  giv,en  variation  or  tendency  can  be  developed.  If  the  experi- 
ence of  the  future  confirms  that  of  the  past,  it  would  seem  to 
furnish  a  formidable  objection  to  Darwin's  conception  that  in 
wild  nature  fluctuating  variations  may  be  accumulated  until  a 
new  species  is  formed. 

Selection  of  mutations.  Darwin  was  aware  of  the  fact  that 
sports,  or  mutations,  occasionally  arose  in  nature,  but  he  did  not 
think  that  they  occurred  with  sufficient  frequency  or  in  sufficient 
numbers  to  be  of  great  importance  in  the  evolution  of  wild 
species.  The  experiments  and  writings  of  De  Vries,  however, 
and  the  experience  of  breeders  and  botanists  who  have  been  stim- 
ulated by  his  discoveries,  seem  to  indicate  that  mutations  may  be 
a  more  important  factor  in  evolution  than  Darwin  supposed. 
Variation  and  selection  would  here,  as  in  evolution  through 
fluctuating  variations,  be  the  fundamental  principles  concerned 
in  the  origin  and  perpetuation  of  improved  varieties  or  species. 
"  Species,"  says  De  Vries,  "  are  derived  from  other  species  by 
means  of  sudden  small  changes,  which,  in  some  instances,  may 
be  scarcely  perceptible  to  the  inexperienced  eye.  From  their 
first  appearance  they  are  uniform  and  constant  when  propagated 
by  seed  ;  they  are  not  connected  with  the  parent  species  by  inter- 
mediates and  have  no  period  of  slow  development  before  they 
reach  the  full  display  of  their  characters."  Some  young  species 
will  be  better  fitted  for  their  life  conditions  than  others,  and  the 
struggle  for  life  will  induce  a  selection  among  them  by  which 
the  fittest  survive.  And  again :  "  Thus  we  come  to  the  conclu- 
sion that  natural  selection  is  as  active  as  Darwin  assumed  it  to 
be,  and  is  as  preeminent  a  factor  in  evolution.  It  causes  the 


PLANT  BREEDING  AND  EVOLUTION  211 

survival  of  the  fittest;  but  it  is  not  the  survival  of  the  fittest 
individuals,  but  that  of  the  fittest  species^by  which  it  guides  the 
development  of  the  plant  and  animal  kingdoms." 

SUMMARY 

Whether  we  accept  the  theory  of  Darwin  or  that  of  De  Vries,  it 
will  be  evident  to  the  student  that  the  production  and  survival  of 
improved  races  of  plants  has  proceeded  along  the  same  general  lines 
in  culture  and  ID  wild  nature.  Variations,  either  of  the  nature  of 
fluctuating  variations  or  of  constant  mutations,  are  necessarily  the 
starting  points  for  the  origin  of  a  new  species  or  variety.  Selection 
of  the  best-adapted  plants  for  a  given  human  purpose,  or  to  fit  the 
conditions  of  a  given  environment,  are  then  made  by  man,  or  by 
nature,  through  competition  and  struggle. 

The  survival  of  the  fittest,  in  nature,  means  that  certain  plants 
possess  characters,  produced  by  variation,  which  enable  them  to 
live  in  certain  habitats  in  competition  with  their  neighbors,  while 
other  plants  of  the  same  species  or  kin  are  unable  to  do  so.  The 
survival  of  the  fittest,  in  culture,  means  that  those  plants  live  and  per- 
petuate their  kind  which  most  nearly  meet  the  needs  and  desires  of 
man  in  different  countries  and  regions.  They  are  not  necessarily 
the  plants  best  adapted  to  wild  life  in  any  given  environment,  since 
man  creates  a  new  environment  for  the  plants  of  his  choice  by  irri- 
gation, by  fertilizers,  and  by  cultivation.  In  other  words,  man 
adapts  the  environment  to  the  plants  of  his  choice. 

The  double  roses,  chrysanthemums,  and  peonies  are  not  the  fittest 
plants  to  survive  in  their  native  habitats.  They  survive  under  cul- 
ture because  they  are  propagated  and  cultivated  in  an  artificial 
environment  created  by  the  horticulturist  and  the  plant  breeder. 
Despite  these  discrepancies  in  the  final  results,  evolution  in  nature 
and  plant  improvement  in  culture  are  based  upon  the  same  general 
laws  of  variation  and  heredity,  and  are  dependent  upon  the  same 
general  principle  of  selection. 


CHAPTER  XI 

HISTORICAL  DEVELOPMENT  OF  BOTANY  AND  THE 
BIOLOGICAL  SCIENCES 

Biology  in  its  broadest  sense  includes  all  studies  pertaining  to 
the  form,  structure,  and  activities  of  living  plants  and  animals. 
In  many  American  colleges  and  universities,  however,  the  term 
biology  is  used  in  a  restricted  sense,  to  designate  an  introductory 
course  in  biological  science  for  beginning  students.  In  such  an 
introductory  course  the  general  facts  and  principles  of  the  science 
are  presented  comprehensively  from  the  standpoint  of  both  plant 
and  animal  life,  in  order  either  to  lay  a  foundation  for  more  ex- 
tended studies  in  botany  and  zoology  or  to  serve  as  a  basis  for 
general  reading  and  culture.  In  the  actual  work  of  the  teacher 
or  the  investigator  it  has  been  found  necessary,  however,  to  sub- 
divide the  science  of  biology  into  botany,  the  science  of  plants, 
and  zoology,  the  science  of  animals.  These  two  sciences  are  again 
subdivided  into  many  lesser  branches,  often  termed  sciences,  since 
no  one  mind  can  any  longer  master  more  than  a  small  corner  of 
the  ever- widening  field  of  biology.  This  subdivision  of  biological 
science  into  its  smaller  branches  has  come  about  in  a  natural  way 
as  the  knowledge  of  animals  and  plants  has  gradually  increased 
by  study  and  research. 

Classification,  systematic  biology,  or  taxonomy.  The  early 
studies  of  both  botanists  and  zoologists  were  made  in  an  attempt 
to  classify  plants  and  animals  into  groups  based  upon  external 
resemblances.  These  early  attempts  at  grouping  organisms  gave 
rise  to  a  definite  branch  of  biology  now  known  as  classification, 
systematic  biology,  or  taxonomy,  represented  in  our  many  man- 
uals of  botany  and  zoology,  which  are  intended  to  enable  students 
to  secure  a  clearer  idea  of  the  relationships  and  general  attributes 
of  common  plants  and  animals.  In  botany  Asa  Gray  was  the 
greatest  early  systematic  botanist  in  the  United  States,  and  we  still 

212 


HISTORICAL  DEVELOPMENT  OF  BOTANY         213 

have  Gray's  manuals  as  our  principal  aids  to  a  better  knowledge 
of  the  classification  of  our  common  wild  flowering  plants.  Since, 
however,  a  correct  classification  of  any  group  of  organisms,  such 
as  oaks,  grasses,  or  fishes,  is  based  not  only  upon  external  char- 
acters but  upon  details  of  structure  and  development  as  well,  it 
is  quite  obvious  that  we  can  never  make  a  reliable  classification 
of  any  group  of  living  things  until  we  have  a  more  or  less 
complete  knowledge  of  the  comparative  structure  and  develop- 
ment of  the  individual  members  of  such  a  group.  Such  a  knowl- 
edge of  the  great  groups  of  animals  and  plants  which  are  now 
classed  together  is  not  yet  available,  and  classification,  although 
the  oldest,  is  consequently  a  more  or  less  artificial  branch  of 
biological  science  to-day. 

Morphology.  As  the  name  indicates,  morphology  is  the  science 
of  the  form  of  the  animal  or  plant  body ;  but  it  has  come  to 
include  the  structure  and  development,  as  well  as  the  form,  of 
living  organisms.  It  may  include  simply  the  study  of  the  gen- 
eral form,  structure,  and  development  of  a  single  organism  or 
it  may  refer  to  a  comparative  study  of  the  main  organs,  such 
as  leaves,  stem,  roots,  and  flowers,  of  a  group  of  related  organ- 
isms. Applied  to  such  organisms  as  pine  trees  and  their  allies, 
it  would  comprehend  the  common  conelike  habit  of  pines  and 
the  gross  structure  and  form  of  their  branches,  leaves,  roots,  and 
reproductive  cones.  These  factors,  taken  together,  determine  the 
characteristic  form,  appearance,  and  mode  of  reproduction  of 
members  of  the  pine  family.  Since  morphology  has  come  to 
include  the  structure  and  development  as  well  as  the  form  of 
organisms  and  their  parts,  special  terms  are  now  used  to  desig- 
nate the  more  detailed  studies  of  structure  and  development 
which  have  grown  out  of  the  older  and  grosser  morphology. 

Anatomy.  The  term  anatomy  is  usually  applied  to  the  more 
general  studies  of  the  structure  of  the  tissues,  and  organs  of 
both  animals  and  plants.  In  plants  anatomy  is  necessarily 
microscopic,  while  in  animals  it  includes  the  grosser  observa- 
tions of  the  organs  and  tissues  which  can  be  acquired  by  dis- 
sections. Thus,  the  anatomy  of  a  pine  tree  would  include  the 
microscopic  structure  of  sections  of  the  trunk,  leaves,  branches, 


214  GENERAL  BOTANY 

and  reproductive  organs,  while  the  anatomy  of  a  cat  would  include 
the  gross  structure  of  the  tissues  and  the  main  organs  of  the  body. 

Histology.  In  plants  histology  is  easily  confused  with  anatomy, 
since  each  includes  the  microscopic  structure  of  the  tissues  and 
organs,  although  histology  in  plants  comprehends  more  detailed 
microscopic  structures  of  the  cell  units  which  make  up  the  tis- 
sues and  organs  than  is  the  case  in  plant  anatomy.  In  animals, 
however,  histology  is  distinct  from  anatomy  as  it  is  generally 
understood,  since  histology  in  animals  means  the  microscopic 
structure  of  the  cell  units  of  the  parts  of  the  animal  body,  which 
are  observed  in  gross  in  anatomical  studies. 

Cytology.  Cytology  is  the  newest  of  the  branches  of  biology 
yet  mentioned,  and  includes  the  minute  microscopic  structure 
of  the  cell  units  of  living  organisms  and  also  of  the  ultimate 
structure  of  living  matter. 

Embryology.  As  the  term  is  commonly  understood,  embryology 
applies  to  the  processes  of  development  of  an  organism  from  the 
early  divisions  of  the  fertilized  egg  to  the  adult  stage,  in  which  the 
various  organs  and  tissues  have  reached  their  ultimate  form  and 
structure.  Embryology,  like  morphology,  may  include  simply  the 
processes  and  stages  in  the  development  of  an  individual  organism, 
or  it  may  be  comparative  in  its  nature  and  include  a  comparison 
of  the  developmental  processes  of  a  series  of  related  organisms. 

Physiology.  Opposed  to  morphology  and  its  subdivisions  is 
physiology,  which  deals  with  the  functions  of  organs  and  parts 
of  single  individuals  or  with  the  activities  of  organisms  in  gen- 
eral. The  field  of  physiology  is  rapidly  extending  to-day  into 
the  older  fields  of  biology,  as  is  indicated  by  the  newer  studies 
in  experimental  morphology,  ecology,  embryology,  and  evolu- 
tion. In  all  of  these  new  fields  of  study  the  facts  accepted  by 
older  systematic  and  morphological  biology  are  being  tested  by 
physiological  experiments  and  observations. 

Ecology.  The  newest  of  the  main  subdivisions  of  biological 
science  is  ecology,  which  comprehends  the  relation  of  plants 
and  animals  to  their  environment  and  the  relation  which  exists 
between  form,  structure,  and  environmental  influence.  Applied 
to  plants  it  includes  the  effects  of  light,  heat,  moisture,  and  soil 


HISTOEICAL  DEVELOPMENT  OF  BOTANY         215 

on  the  ultimate  form,  structure,  and  reproductive  processes  of 
plants  in  a  given  home,  or  place  of  growth.  In  its  wider  appli- 
cations ecology  includes  the  plant  associations  in  different  local- 
ities, and  the  way  in  which  environment  affects  migration  and 
the  distribution  of  organisms  on  the  earth's  surface.  In  addition 
to  these  more  or  less  definitely  conceived  branches  of  biology 
there  are  many  others,  of  great  theoretical  and  practical  impor- 
tance to  mankind,  which  are  difficult  to  classify  on  account  of 
their  close  interrelation  and  overlapping. 

Evolution.  Ever  since  the  time  of  Charles  Darwin  the  term 
evolution  has  included  all  studies  in  the  method  of  origin  and 
development  of  the  higher  forms  of  plant  and  animal  life  from 
lower  forms.  The  idea  of  evolution,  since  its  acceptance,  has 
determined  the  aims  and  methods  of  study  in  all  branches  of  bio- 
logical science,  and  may  properly  be  said  to  dominate  all  studies 
in  comparative  morphology,  embryology,  physiology,  and  ecology. 

Closely  connected  with  evolution  are  such  subjects  as  the 
dispersal  and  distribution  of  organisms,  and  the  modern  studies 
in  heredity,  variation,  and  breeding,  now  being  included  under 
genetics. 

Industrial  and  applied  biology.  Under  industrial  and  applied 
biology  is  included  the  application  of  such  purely  scientific  bio- 
logical pursuits  as  bacteriology,  mycology  (the  study  of  fungi), 
physiology,  and  genetics  to  medicine,  sanitary  science,  agricul- 
ture, horticulture,  and  forestry.  This  application  of  economic 
biological  principles  to  the  problems  of  agriculture,  horticulture, 
and  forestry  is  mainly  centered  around  the  departments  of  agri- 
culture and  forestry  of  the  national  and  state  governments. 

Plant  and  animal  pathology.  The  most  important  of  these 
new  applications  of  biology  to  the  diseases  of  domesticated 
animals  and  cultivated  plants  are  plant  and  animal  pathology. 
Millions  of  dollars  are  saved  to  the  state  and  national  govern- 
ments annually  as  a  result  of  the  efforts  now  being  made  to 
counteract  the  effect  of  fungous  and  bacterial  diseases  in  plants 
and  animals  which  are  economically  important  to  man.  Plant 
and  animal  pathology  are  therefore  important  adjuncts  to  the 
new  scientific  agriculture  and  forestry. 


216 


GENERAL  BOTANY 


Plant  and  animal  breeding.  Another  important  field  for  re- 
search and  experiment,  closely  allied,  as  already  indicated,  to  the 
modern  studies  being  made  in  genetics  and  heredity,  is  plant  and 
animal  breeding.  This  attempt  to  produce  new  and  better  forms 
of  plant  and  animal  life  for  man's  use  has  followed  closely  upon 
the  generalizations  of  Darwin,  Mendel,  and  De  Vries  concerning 
the  nature  of  evolution,  variation,  and  heredity. 

The  field  of  biology  is  thus  an  ever-widening  one,  in  which 
the  purely  scientific  and  theoretical  studies  of  the  scientist  are 
being  constantly  applied  to  arts  and  professions  which  are  imme- 
diately concerned  with  the  welfare  and  progress  of  society. 

The  following  table  will  help  the  student  to  comprehend 
clearly  the  scope  of  biology  as  outlined  above  and  to  understand 
the  interrelations  of  its  various  branches  and  applications. 


Botany : 
Science  of 
plants 


BIOLOGY  4 


Zoology : 
Science  of 
animals 


Morphology : 
Science  of 
form  and 
structure 


'  Embryology :  Science  of  develop- 
ment 
Anatomy :  Gross  structure 

Tissue  and  cell  struc- 


Morphological 
and   physio-  - 
logical 


Histology 
ture 

Cytology  :  Structure  of  the  cell 

Classification  :  Grouping  based 
on  relationship 

Distribution 

Ecology :  Adaptation  to  environ- 
ment 

Evolution  :  Origin  of  species  by 
gradual  accumulated  variation, 
or  change 

Genetics  :    Experimental   evolu- 
tion and  heredity 
^  Physiology  :  Science  of  function  and  behavior 


Applied  and  Indus- 
trial Biology 


Breeding    *| 

Pathology  /  Plant  and  animal 

Agriculture ") 

^  f  Economic  and  industrial 

t  orestry       J 

Horticulture  "1 

Medicine        [•  Arts  and  professions  related  to  biology 

Sanitation 


PART  II.     THE  PLANT  GROUPS 


CHAPTER  XII 

THE  ALGJE 

The  algae  are  the  simplest  of  the  green  plants  and  constitute, 
with  the  fungi,  the  group  known  as  thallophytes,  -or  plants  with- 
out true  roots,  stems,  and  leaves.  The  fresh- water  algae  inhabit 
fresh-water  streams,  lakes,  and  ponds,  as  well  as  wet  banks  and 
the  bark  of  trees.  They  usually  occur  in  simple  colonies  in.  which 
the  cells  unite  to  form  cell  chains,  nets,  or  spherical  aggregates. 
Like  the  higher  plants,  the  form  which  the  colony  assumes  is 
usually  closely  related  to  its  needs  and  to  its  mode  of  life. 

The  immense  growth  of  algae  in  ponds  and  streams,  due  to 
their  rapid  methods  of  reproduction,  is  often  of  the  greatest  im- 
portance in  producing  food  for  fish  and  minute  aquatic  animals. 
The  term  plankton  is  applied  to  the  great  mass  of  minute  living 
algae  and  other  organisms  floating  on  the  surface  of  our  lakes  and 
ponds  in  summer.  This  plankton  is  of  vital  importance  to  the 
great  fisheries  which  supply  European  countries  and  our  own  coun- 
try with  fish  food.  The  Illinois  State  Survey,  under  Dr.  Forbes, 
estimated  that  the  Illinois  River  plankton  produces  annually 
about  150,000,000  pounds  of  fish  food,  and  it  has  been  estimated 
that  the  plankton  of  the  Rhone  River  comprises  8000  different 
species  of  microscopic  plants  and  800  species  of  microscopic 
animals.  This  immense  number  of  forms  is  largely  due  to  then- 
rapid  methods  of  asexual  reproduction  during  the  summer  months. 

Water  supplies  are  also  affected  by  the  very  rapid  multiplica- 
tion of  algse  during  the  warm  season.  In  this  case  the  diffi- 
culty is  due  not  so  much  to  the  dangerous  nature  of  the  algae 
themselves  as  to  the  fact  that  their  decay  furnishes  food  for 
bacteria  which  are  inimical  to  life  if  taken  into  the  system  with 
drinking  water.  The  presence  of  algae  in  any  considerable  abun- 
dance in  a  water  supply  is  therefore  a  sign,  and  an  indirect  cause, 
of  danger  to  those  using  the  water. 

219 


220  GENERAL  BOTANY 

The  marine  algae  inhabit  the  salt  water  of  the  ocean  and  the 
brackish  Avater  of  salt  marshes  and  ponds.  The  lowest  and  sim- 
plest forms  resemble  closely  the  green  fresh- water  algae,  to  which 
they  are  closely  related.  Among  the  higher  forms,  however,  the 
plant  body  attains  to  much  greater  complexity  than  any  known 
fresh- water  species,  and  the  colors  are  often  very  striking,  ranging 
from  bright  reds  to  dull  browns  and  greens.  Some  larger  brown 
algae  may  attain  to  a  length  of  a  hundred  feet  or  more,  and 
are  attached  at  the  base  by  large  and  strong  holdfasts  resem- 
bling roots.  These  large  forms  are  usually  long,  flattened,  strap- 
shaped  bodies  which  float  out  upon  the  surface  of  the  water  by 
means  of  special  air  cavities,  or  spaces  within  their  tissues,  called 
floats.  The  form  thus  assumed  and  the  floating  habit  are  both 
adapted  to  the  needs  of  these  organisms  in  their  manufacture 
of  foods  by  photosynthesis.  The  red  and  brown  colors  are  due 
to  pigments  secreted  in  the  chloroplasts,  which  partially  or 
wholly  mask  the  green  chlorophyll  pigment ;  but  the  significance 
of  these  additional  pigments  is  not  yet  fully  understood. 

PROTOCOCCUS 

Protococcus  occurs  in  the  form  of  single  cells  or  of  loose 
colonies  growing  on  bark  and  moist  surfaces  of  all  kinds,  such 
as  boards,  stones,  brick  walls,  etc.  Its  genetic  relationship  to 
the  other  algae  is  not  certainly  known,  since  it  is  a  xerophytic 
type  whose  form  and  life  history  has  become  greatly  modified 
during  its  gradual  adaptation  to  its  present  habitat.  It  is  used 
here  for  our  initial  study  of  the  simplest  fresh-water  algae  on 
account  of  its  wide  distribution  and  availability  for  class  use. 

Structure  and  mode  of  life.  Each  cell  of  Protococcus  (Fig.  115) 
has  a  cellulose  cell  wall  which  contains  a  many-lobed  chloroplast 
and  a  nucleus  embedded  in  the  cell  cytoplasm.  The  plants  grow 
in  masses,  forming  green  incrustations  on  the  surfaces  which 
they  inhabit.  They  are  essentially  air  plants,  absorbing  carbon 
dioxide  and  oxygen  from  the  air  and  taking  up  soil  salts  from 
the  dust  particles  dissolved  in  the  rain  or  dew  which  moistens 
their  surface.  In  this  way  they  are  able  to  subsist  without  the 


THE  ALGLE  221 

complex  organs  which  are  always  associated  in  higher  plants 
with  the  processes  of  nutrition.  The  single  cell  of  which  the 
plant  body  of  Protococcus  is  composed  is  thus  a  very  general 
cell  as  regards  its  life  functions,  in  comparison  to  the  highly 
specialized  wood,  bark,  or  leaf  cells  of  higher  plants.  These 
cells  have  become  differentiated  to  perform  particular  kinds  of 
work  in  a  complex  organism,  like  a  single  worker  in  a  large 
factory.  The  Protococcus  cell,  on  the  contrary,  is  more  like  the 
general  workman,  since  it  is  able  to  perform  all  of  the  life  func- 
tions by  means  of  its  single  highly  differentiated  protoplast. 

All     of    the    simpler 

,  ,       ^  Cytoplasm 

algse  resemble  Proto- 
coccus in  this  respect ; 
that  is,  in  being  com- 
posed of  cells  which 
are  highly  organized 
but  independent  of  FIG.  115.  Protococcus 

each  Other  in  the  per-      «,  single  Protococcus  plant ;  b,  division  of  the  plant 

f ormance  of  their  life     into  two  cells  by  f u  di™ion  I  c' four  cells  formed 

by  cell  division 

functions. 

Reproduction.  In  Protococcus  reproduction  takes  place  solely  by 
the  vegetative  method.  Its  reproduction  is  thus  comparable  to 
that  of  higher  plants  which  reproduce  by  means  of  buds,  bulbs, 
and  runners.  When  Protococcus  is  about  to  reproduce  vegetatively, 
each  cell  divides  into  two  daughter  cells  which  are  furnished  with 
one  half  of  the  original  protoplast,  plastid,  and  nucleus.  This  cell 
division  takes  place  by  mitosis  in  a  manner  similar  to  that 
already  described  for  root-tip  cells.  The  daughter  cells  thus 
formed  then  divide  again,  and  this  process  may  be  repeated 
many  times.  The  cell  colonies  thus  produced  usually  remain 
united  for  a  time,  forming  the  granular  incrustations  commonly 
seen  on  trees  and  stones  inhabited  by  Protococcus.  These  minute 
cell  colonies  ultimately  separate  into  individual  plants,  which 
are  light  enough  to  be  disseminated  by  the  wind.  Water  may 
.also  be  a  factor  in  the  distribution  of  Protococcus  where  they  are 
located  on  soil  or  rocks.  The  above  facts  account  for  the  wide 
dissemination  of  Protococcus  in  nature. 


222  GENEKAL  BOTANY 


CHLA  M  YDOMONA  S 

In  the  genus  Chlamydomonas,  the  species  of  which  are  common 
in  stagnant  water,  each  plant  (Fig.  116)  is  a  single  cell  with  a 
thin  cell  wall  through  which  two  delicate  protoplasmic  filaments, 
called  flagella,  can  be  seen  to  protrude.  These  flagella  are  minute 
extensions  of  the  outer  layer  of  the  protoplast  of  the  Chlamydo- 
monas  cell,  and  it  is  by  the  contractile,  whiplashlike  movements 
of  these  flagella  that  the  organism  is  propelled  through  the  water. 
These  movements  are  often  seen  to  be  toward  a  source  of  light, 
indicating  that  the  organism  is  sensitive  to  light  of  different  inten- 
sities. This  sensitiveness  is  thought  to  be  located  in  a  brick-red 
(  spot,  erroneously  called  the  eyespot,  which 

contains  a  brick-red  coloring  matter  by  which 

Chlamydomonas 
and  its  near  rel- 
atives are  often 
^         ^^r        ^    v  W        VOr     recognizable. 

Ihe  plant        Gamete  Conjugation  Zygotes  o  i       ,  • 

formation  Reproduction 

FIG.  116.   Sexual  reproduction  of  Chlamydomonas  takes  place,  as 

in  Protococcus, 

by  cell  division ;  but  the  cell  division  in  Chlamydomonas  results 
in  a  cleavage  of  the  protoplast  within  the  mother-cell  wall  into 
two  or  more  separate  protoplasts,  which  then  round  up,  form 
flagella,  and  develop  either  asexual  reproductive  cells,  called 
zoospores,  or  sexual  cells,  called  gametes.  The  gametes  are  fre- 
quently produced  in  greater  numbers  than  the  zoospores,  and 
hence  are  smaller.  The  gametes  are  liberated  in  the  water  by  the 
rupture  of  the  cell  wall  of  the  original  Chlamydomonas  cell,  and 
conjugate  in  pairs  to  form  a  zygote  cell.  This  zygote  cell  then 
enlarges,  secretes  a  protective  cell  wall,  and  undergoes  a  period 
of  rest.  It  germinates  under  favorable  conditions  to  form  asexual 
zoospores  similar  to  the  gametes  in  appearance,  which  enlarge 
to  form  new  free-swimming  plants.  In  some  species  the  gametes 
are  of  unequal  size,  foreshadowing  the  differentiation  in  the  size 
of  gametes  which  obtains  in  higher  forms  of  algae.  In  asexual 
reproduction  the  zoospores  simply  enlarge  and  form  a  new  plant. 


THE  ALGJE  223 

The  free-swimming  forms  of  algse,  like  Chlamydomonas,  are 
undoubtedly  more  primitive  types  of  plants  than  nonmotile 
forms  like  Protococcus,  which  have  apparently  become  adapted 
to  a  dry  habitat  with  the  loss  of  motility. 


SPIROGYRA 

Despite  its  specialized  character,  Spirogyra  has  been  selected 
as  the  first  type  of  a  filamentary  alga  to  be  studied,  on  account 
of  its  availability  and  the  ease  with  which  its  cellular  structure 
and  reproductive  processes  can  be  demonstrated.  It  forms  a 
considerable  part  of  the  green  scum  frequently  seen  on  the 
surfaces  of  stagnant  pools  in  the  summer  and  is  easily  recog- 
nized by  its  slippery  feeling,  due  to  the  gelatinization  of  the 
outer  layer  of  the  cell  walls  of  its  constituent  cells. 

Structure.  Each  cell  of  the  filament  is  a  cylinder  with  a 
cellulose  wall  and  a  thin  cytoplasmic  sac  surrounding  a  large 
central  vacuole.  The  large  nucleus  and  its  nucleolus  are  sus- 
pended in  the  center  of  each  cell  by  strands  of  cytoplasm  ex- 
tending from  the  cytoplasm  surrounding  the  nucleus  to  the 
cytoplasmic  sac  (Fig.117,  A).  The  nucleus  is  thus  bathed  by 
the  nutrient  cell  sap  of  the  water  vacuole. 

Unlike  the  cells  of  higher  plants,  there  is  no  difference 
between  the  cells  composing  the  filaments  of  Spirogyra,  and  no 
differentiation  into  tissues  or  organs.  Each  cell,  therefore,  like 
the  independent  cells  of  Protococcus,  is  capable  of  performing 
all  of  the  functions  of  life.  Absorption,  photosynthesis,  diges- 
tion, assimilation,  and  respiration  are  thus  carried  on  by  each 
cell  independent  of  all  the  others.  It  follows  that  if  Spiro- 
gyra filaments  are  broken  up  by  the  waves  so  as  to  separate  a 
cell  or  a  group  of  cells  from  its  fellows,  such  cells  or  cell  groups 
will  grow  and  produce  new  plants  by  repeated  division  and 
elongation.  Since  each  cell  performs  all  of  its  own  functions,  we 
shall  expect  to  find  its  different  cell  structures  highly  complex. 
This  is  particularly  true  of  the  chloroplasts,  which,  in  addition 
to  the  function  of  photosynthesis,  serve  as  a  temporary  store- 
house for  the  starch  formed  by  it  during  the  hours  of  daylight. 


224 


GENERAL  BOTANY 


The  chloroplast  is  by  far  the  most  striking  structure  in  the 
cells  of  Spirogyra,  when  viewed  with  a  compound  microscope. 
These  chloroplasts  are  in  the  form  of  bands,  one  or  more  in  each 
cell  (Fig.  117),  wound  spirally  around  the  cell  just  beneath  the 
cell  wall.  Each  chloroplast  is  in  reality  a  differentiated  portion 
of  the  cytoplasmic  sac  and  corresponds,  therefore,  in  origin  and 

substance,  to  the  granular  plas- 
tids  of  higher  plants.  In  sections 
of  Spirogyra  cells  (Fig.  118) 
these  bandlike  chloroplasts  are 
seen  to  be  thickened  and  to 
project  as  ridges  of  cytoplasm 
into  the  cell  vacuole  slightly 
more  than  the  intervening  por- 
tions of  the  sac. 

Under  proper  magnification 
each  chloroplast  band  is  also 
seen  to  be  dotted  at  regular 
intervals  with  a  series  of  disk- 


-P 


•n 


FIG.  117.   Cell  structure  of  Spirogyru 

A,  single  cell  of  Spirogyra  with  nucleus 
(ri)  and  spiral  chloroplast  containing 
pyrenoids  (p) ;  B,  cell  with  the  cyto- 
plasmic sac  contracted  by  treatment  with 
salt  solution ;  C,  pyrenoid  in  a  small  por- 
tion of  a  chloroplast.  From  Bergen  and 
Davis's  "  Principles  of  Botany  " 


shaped  bodies  called  pyrenoids, 
the  entire  series  of  pyrenoids 
being  connected  by  a  delicate 
strand  of  cytoplasm  which  is 
devoid  of  green  pigment.  Upon 
investigation  with  the  high 
power  of  the  microscope  the 
pyrenoid  is  found  to  be  differ- 
entiated into  a  darker  central  granule  and  a  lighter  surrounding 
sheath,  called  the  starch  sheath.  The  function  of  the  central 
granule  is  not  definitely  known,  but  the  starch  sheath  is  a 
storage  place  for  the  excess  of  starch  not  used  by  the  cell  dur- 
ing the  daylight,  when  the  chloroplast  is  forming  sugar  and 
starch  by  photosynthesis.  At  night  this  starch  is  digested  or 
transformed  into  sugar  and  is  used  for  the  nutrition  of  the  cells 
during  the  hours  when  photosynthesis  is  not  going  on.  The 
starch  sheath  is  therefore  a  temporary  storage  structure  of  the 
cell  and  of  the  chloroplast.  The  above  function  of  the  pyrenoid 


THE 


225 


can  be  tested  by  treating  some  plants  exposed  to  light  and  some 
kept  in  darkness  with  iodine  solution.  The  plants  exposed  to 
light  will  give  the  blue  reaction  for  starch  in  the  starch  sheath ; 
the  plants  which  have  been  in  darkness  for  some  hours  will  fail 
to  give  this  test,  showing  that  the  stored  starch  has  been  digested 
and  used.  In  this  case,  if  the  plants  are  reexposed  to  light 
they  will  be  found  to  form  starch  again  in 
a  very  few  minutes. 

Physiology.  Spirogyra,  like  Protococcus, 
is  not  materially  different  from  higher 
plants  as  regards  the  essential  processes  of 
nutrition,  cell  division,  and  reproduction. 
The  cell  sap  contains  an  abundance  of 
dissolved  inorganic  food  materials,  which 
the  cells  readily  absorb,  like  a  root-hair  cell, 
by  osmosis.  Carbon  dioxide  and  oxygen 
are  likewise  easily  obtained  from  the  water 
in  which  the  plants  float,  or  it  may  be 
obtained  directly,  from  the  air  when  large 
masses  of  these  algye  rise  to  the  surface  of 
ponds  or  lakes,  buoyed  up  by  the  gas  bub- 
bles liberated  during  photosynthesis. 

Digestion  of  the  excess  starch  formed 
and  temporarily  stored  in  the  pyrenoids 
during  the  day  is  known  to  take  place  at 
night,  as  in  the  leaf  cells  of  higher  plants. 
Since  it  is  also  known  that  the  cells  of 

Spirogyra  usually  divide  and  elongate  during  the  night,  it  is 
probable  that  this  is  a  period  of  active  assimilation  and  respira- 
tion in  the  individual  cells  of  the  filaments. 

Through  the  above  processes  Spirogyra  cells  grow  and  attain 
the  maximum  size  fixed  for  each  species  of  Spirogyra  plant. 
When  this  maximum  size  is  reached,  each  cell  may  divide  and 
form  two  new  cells  by  a  division  wall  which  is  laid  down  by  the 
cytoplasm  across  the  middle  of  the  cell.  This  cell  division  takes 
place  in  the  following  manner:  The  new  dividing  wall  first 
forms  a  thickening,  ringlike  ridge  of  cellulose  on  the  old  cell 


-Cell  wall 
Pijrenoid 
Nucleus 
Cytoplasm 
Vacuole 


cell 
icall 


FiG^llS.      Cell  struc- 
ture and  cell  division  in 
Spirogyra 

The  cells  are  shown  as 
they  appear  in  a  longi- 
tudinal section.  Observe 
the  relation  of  pyrenoids, 
cytoplasm,  and  nucleus 


226  GENEBAL  BOTANY 

wall  at  the  middle  of  the  mother  cell  (Fig.  118).  As  the  ridge 
grows  by  the  deposit  of  new  cellulose  on  its  inner  edge  it 
projects  farther  and  farther  into  the  cell  like  a  thin  disk,  or 
plate,  except  that  the  center  of  the  disk  is  not  yet  completed. 
As  a  result  of  this  growth  of  the  cell  wall  the  protoplasmic  sac 
is  pushed  in  and  constricted  before  the  advancing  wall.  Finally 
the  entire  sac  is  cut  off  by  the  completion  of  the  disk,  and  the 
new  wall  forms  a  complete  plate  across  the  old  cell,  dividing  it 
into  two  new  daughter  cells.  During  this  process  the  nucleus 
divides  into  two  equal  parts,  each  of  which  organizes  a  new 
nucleus  in  one  of  the  daughter  cells.  The  daughter  cells  and 
their  nuclei  then  grow  to  the  size  of  the  mother  cell,  and  in  this 
manner  the  filaments  elongate. 

Reproduction.  Vegetative  reproduction  occurs  when  the  delicate 
filaments  are  broken  up  by  the  waves  or  are  separated  into  single 
cells  or  cell  fragments.  Since  each  cell  of  Spirogyra  is  capable 
of  maintaining  life  and  growth  independently  of  the  other  cells 
of  the  filament  it  follows  that  individual  cells  or  fragments  of 
filaments  are  able  to  elongate  into  new  plants  and  thus  to 
multiply  the  number  of  individuals  indefinitely. 

Sexual  reproduction  takes  place  regularly  either  in  the  early 
spring  or  in  the  fall,  before  the  plants  enter  upon  the  winter 
period  of  rest.  Bright  sunshine  and  abundant  rainfall  appear  to 
be  factors  in  inducing  Spirogyra  to  resort  to  sexual  reproduc- 
tion, while  the  opposite  conditions,  with  abundant  nutriment  in 
the  water  in  which  the  plants  live,  tend  to  prolong  the  growth 
period  for  vegetative  reproduction.  This  is  doubtless  due  to  a 
stimulus  exerted  by  light  and  soluble  food  salts  upon  the  deli- 
cate protoplasts  of  the  filaments,  which  respond  by  initiating 
reproductive  or  vegetative  growth  processes,  just  as  the  cells  of 
a  root  may  respond  by  tropistic  movements  to  the  influence 
of  gravity. 

G-ametophyte  is  the  term  applied  to  the  plant  filaments  of  Spiro- 
gyra which  reproduce  and  form  new  offspring  by  means  of 
gametes.  When  such  plants  are  about  to  reproduce  sexually,  two 
filaments  which  lie  near  each  other  become  united  by  tubular  out- 
growths formed  by  protrusions  from  their  cells  (Fig.  119,  a). 


THE  ALG.E 


227 


These  protrusions  finally  meet  and  unite  to  form  tubes  connecting 
the  cells  of  the  conjugating  filaments  in  pairs  (6).  When  the  cell 
wall  between  the  united  protrusions  is  absorbed,  the  protoplasts 
of  each  pair  of  cells  come  into  immediate  contact,  which  initiates 
in  them  certain  changes  resulting  in  their  conversion  into  the 
female  and  male  gametes.  This  conjugation  of  the  filaments 
thus  marks  the  initial  stage  of  sexual  reproduction  in  Spirogyra. 

G-amete  formation  is  a  very  simple  process  consisting  in  the 
shrinkage  of  entire 
protoplasts  in  the 
cells  of  two  filaments 
united  by  conjugat- 
ing tubes.  Each 
gamete  thus  con- 
sists of  the  original 
cytoplasm,  nucleus, 
and  chloroplast  of  an 
ordinary  Spirogyra 
cell  contracted  into 
a  rounded  mass  of 
protoplasm.  In  most 
species  of  Sjoirogyra, 
gametes  are  formed 
in  one  of  the  conju- 
gating filaments  before  the  protoplast  of  the  other  filament  begins 
to  contract.  Since  these  first-formed  gametes  are  the  ones  which 
move  through  the  conjugating  tubes  during  fertilization  to  unite 
with  the  stationary  gametes  of  the  adjacent  filament,  they  are 
usually  considered  to  be  the  male  gametes.  Such  species  of 
Spirogyra  are  therefore  said  to  be  dioecious,  which  means  that 
one  filament  bears  only  male  gametes  and  the  other  only  female. 

Gametangium  is  a  term  applied  to  the  entire  cell  in  which 
the  protoplast  contracts  to  form  a  gamete.  This  cell  may  be 
termed  either  the  male  or  the  female  gametangium,  according 
as  it  forms  a  male  or  a  female  gamete. 

Fertilization  consists  in  the  union  of  the  motile  male  gamete 
with  the  stationary  female  gamete  of  the  opposite  filament.  In 


FIG.  119.    Sexual  reproduction  in  Spirogyra 

a,  early  conjugation  of  the  filaments  (the  pairs  of  uniting 
cells  are  gametangia  in  each  case)  1^6,  gametes  forming 
and  uniting  in  conjugation  to  form  zygotes;  c,  zygotes 
with  thick  cell  wall,  pyrenoids,  and  conjugate  nucleus; 
d,  germination  of  the  zygote  to  form  a  new  plant 


228  GENERAL  BOTANY 

this  process  the  cytoplasm  of  the  two  gametes  fuse  first,  and  the 
two  gamete  nuclei,  called  prcwudei,  fuse  later  to  form  the  one 
double  conjugate  nucleus..  As  soon  as  the  gametes  unite,  the 
double  cell  formed  by  the  union  of  the  gametes  rounds  up 
and  secretes  a  cell  wall,  which  completes  the  formation  of  the 
zygote  (e).  Since  this  zygote  in  Spirogyra  is  designed  to  carry 
the  plant  over  inclement  periods  of  drought  or  cold,  it  develops 
a  very  heavy  protective  cell  wall,  which  is  divided  into  three 
layers.  The  outer  layer  is  thick  and  protects  the  protoplast  from 
mechanical  injury  and  sudden  changes  of  temperature  ;  the  middle 
coat  is  impervious  to  water  and  prevents  its  loss  from  the  pro- 
toplast during  drought  if  the  pond  or  stream  in  which  it  lives 
happens  to  dry  up  in  summer;  the  inner  coat  is  thin  and  elastic. 
Pyrenoids  are  also  abundant  in  the  mature  zygote  and,  with  the 
aid  of  the  microscope,  are  easily  distinguished  from  the  nucleus 
by  the  central  granule  and  surrounding  starch  sheath. 

After  its  rest  period  germination  of  the  zygote  (c?)  takes  place, 
owing  to  environmental  stimuli.  In  the  germination  process  the 
zygote  absorbs  water,  and  the  swelling  of  its  contents  results  in 
the  rupture  of  the  two  outer  protective  layers  of  the  cell  wall, 
which  allows  the  inner  elastic  layer  to  protrude  and  elongate 
into  a  filament.  This  elongation  of  the  zygote  cell  is  accom- 
panied by  cell  and  nuclear  divisions  which  result  in  an  offspring 
in  the  form  of  a  minute  Spirogyra  plant  in  which  the  chloroplasts 
are  early  reorganized  and  the  nucleus  takes  up  its  central  posi- 
tion in  each  cell.  The  embryo  plant  thus  begins  to  develop  by 
growth  and  cell  division  into  an  adult  Spirogyra  plant,  which 
completes  the  process  of  reproduction,  and  the  new  plant  enters 
upon  a  period  of  independent  vegetative  life. 

LIFE  HISTORIES  OF  ALG^E 

Every  kind  of  organism,  during  its  life,  passes  through  certain 
stages  and  processes  which,  taken  together,  are  termed  its  life 
history.  The  life  history  may  therefore  be  conveniently  considered 
and  graphically  illustrated  as  a  cycle  of  events  in  the  life  of  an 
individual  organism  which  follow  each  other  in  orderly  sequence. 


THE  ALGM  229 

It  is  evident  that  the  simplest  life  history  in  plants  would  be 
presented  by  such  a  simple  organism  as  Protococcus,  which  repro- 
duces by  the  division  of  a  mother  cell  into  two  daughter  cells, 
each  of  which,  by  growth  and  separation,  forms  a  plant  like  the 
mother.  The  very  rapid  reproduction  and  spreading  of  Proto- 
coccus, bacteria,  and  yeasts  is  effected  by  this  mode  of  vegetative 
reproduction. 

The  life  history  of  a  plant  with  sexual  reproduction  alone  or 
with  sexual  reproduction  combined  with  vegetative  reproduction, 
as  in  Spirogyra,  also  presents  a  comparatively  simple  cycle  of 
stages.  In  Spirogyra  the  nutritive  and  growth  phases  of  the  life 
cycle  are  amply  provided  for  by  the  highly  organized  cells  of  the 
filaments,  which  may  also  be  multiplied  by  vegetative  reproduc- 
tion. The  nutritive  phases  usually  continue  throughout  the  sum- 
mer months,  when  food  is  plentiful  in  the  waters  in  which  these 
plants  are  habitually  found.  The  sexual  phase  is  inaugurated  by 
the  formation  of  gametes,  and  terminates  with  the  maturing  of 
the  zygote  resulting  from  the  fertilizing  act.  After  its  normal 
period  of  rest  the  zygote  gives  rise  to  an  offspring  in  the  form  of 
a  new  nutritive  plant,  which  brings  the  life  cycle  back  to  its  start- 
ing point.  In  Chlamydomonas  there  is  added  to  the  vegetative 
and  sexual  phases  of  reproduction  an  asexual  phase,  represented 
by  zoospores.  In  such  plants  the  asexual  zoospore  stage  usually 
has  the  function  of  rapid  reproduction  of  the  species,  while  the 
zygote,  which  results  from  sexual  union,  functions  as  a  resting 
cell  during  inclement  periods  or  seasons,  so  that  there  is  a  divi- 
sion of  labor  between  the  asexual  and  sexual  phases  in  the  life 
history.  The  relation  of  the  different  phases  in  the  life  history  of 
a  plant  may  be  indicated  as  in  the  following  outline  of  the  life 
history  of  Spirogyra.  The  student  should  substitute  simple 
diagrams  for  the  written  designations  of  the  various  stages. 

male  male 

y/gametangia       gametes  \^ 
Gameto-/  \  fertili-  zygote  gameto- 

phytes    \.  Xzation        germination       phyte 

N^     female  female  / 

gametangia       gametes 


230  GENERAL  BOTANY 

VAUCHERIA   (GREEN  FELT) 

Various  species  of  Vaucheria,  or  green  felt,  are  common,  either 
growing  on  wet  soil  in  greenhouses  and  on  muddy  banks  or  float- 
ing free  with  Spirogyra  and  other  algse  on  the  surface  of  ponds 
and  lakes.  The  species  selected  for  our  immediate  study  is  usually 
found  growing  on  very  wet  soil  in  dense  green  masses  or  mats, 
closely  interwoven  by  the  branching  of  the  filaments  making  up 
the  main  portions  of  the  plant  body. 

Sexual  reproduction.  The  gametophyte  of  Vaucheria  sessilis  is 
made  up  of  filaments  like  Spirogyra,  but  these  filaments  are 
highly  branched  and  are  not  divided  by  cell  walls,  so  that  the 
entire  plant  body  of  an  individual  plant  is  composed  of  one  very 
large  multinucleate  cell.  This  long  and  highly  branched  cell  has 
the  same  general  structure  as  that  already  indicated  for  adult  cells 
in  the  higher  plants,  since  it  is  furnished  with  a  cell  wall  of  cellu- 
lose lined  by  a  thin  cytoplasmic  sac,  which  is  kept  in  place  against 
the  cell  wall  by  a  huge  water  vacuole. 

Within  the  cytoplasmic  sac  are  numerous  minute  nuclei 
and  small  granular  chloroplasts,  as  well  as  many  oil  drops, 
which  represent  the  reserve  food  of  the  filament.  This  struc- 
ture of  the  gamete  plant  of  Vaucheria  is  thus  quite  unusual 
among  the  fresh- water  algae  in  that  the  plant  body,  while 
filamentary  and  highly  branched,  is  yet  composed  of  a  single 
multinucleate  cell. 

The  gametangia  and  gametes  (Fig.  120)  are  formed  from 
specialized  branches  of  the  plant  body,  which  arise  at  irregular 
intervals  along  the  filaments  at  the  time  when  sexual  repro- 
duction is  about  to  occur.  These  reproductive  branches  at  the 
outset  resemble  ordinary  young  vegetative  branches,  but  they 
soon  begin  to  differentiate  (a)  into  a  swollen  subspherical 
female  branch  and  a  cylindrical  curved  male  branch.  Each 
subspherical  enlargement'  becomes  filled  with  very  dense  cyto- 
plasm and  with  many  nuclei,  which  flow  into  it  from  the  main 
filament  as  it  increases  in  size.  Sooner  or  later  a  transverse  wall 
cuts  it  off  to  form  a  female  gametangium  (6).  In  this  young 
gametangium  the  cytoplasm  rounds  up  to  form  the  female 


THE  ALG.E 


231 


gamete,  all  but  one  of  the  nuclei  disappear,  and  a  projecting 
beak  is  formed,  filled  with  clear  cytoplasm  free  from  green 
chloroplasts.  With  these  changes  the  gametangium  and  its 
inclosed  gamete  are  made  ready  for  the  reception  of  the  male 
gamete  at  the  time  of  fertilization.  The  male  gametangium, 
which  began  as  a  cylindrical  outgrowth  of  the  reproductive 
branch,  elongates  and  cuts  off  an  end  cell,  which  forms  the  male 
gametangium  (6).  The  dense  granular  cytoplasm  of -the  male 
gametangium,  instead  of  forming  a  single  gamete,  as  in  the 


igomum 

or  female 
gamete 

'ell  wall 
toplasm> 


Anther  idiu 
Male, 

Female     ga 
branch 


FIG.  120.   Sexual  reproduction  in  Vaucheria 

a,  portion  of  a  filament  with  male  and  female  reproductive  branches;  b,  antheridium 

and  oogonium,  with  sperms  and  egg ;  c,  liberation  of  sperms,  and  sperms  swarming 

around  the  receptive  spot  of  the  egg  ;    d,  sperms ;   e,  fertilized  egg,  with  union  of 

male  and  female  pronuclei 

female,  divides  into  a  very  large  number  of  naked  protoplasts, 
each  of  which  becomes  a  motile  male  gamete.  These  minute 
male  gametes  are  finally  liberated  in  the  water  by  the  softening 
and  rupture  of  the  cell  wall  at  the  end  of  the  male  game- 
tangium (e)  and  are  ready  to  effect  a  union  with  the  eggs  in 
the  female  gametangia. 

Fertilization  results  from  the  union  of  the  cytoplasm  and 
nucleus  of  one  of  these  small  male  gametes  with  that  of  the 
large  female  gamete  (Fig.  120,  c).  In  order  to  effect  this 
union  the  cell  wall  at  the  end  of  the  beak  of  the  female  game- 
tangium softens  and  (e)  finally  breaks  down  so  that  a  free  passage 
is  made  for  the  motile  gamete  to  the  cytoplasm  of  the  female 


232 


GENERAL  BOTANY 


gamete.  Many  botanists  hold  that  the  female  gamete  secretes 
some  substance,  such  as  cane  sugar  or  an  organic  acid,  which 
attracts  the  male  gamete  to  the  female  and  thus  insures 
fertilization  by  directing  the  free  motile  male  gamete  to  the 
stationary  egg. 

The  zygote,  or  fertilized  female  gamete,  remains  in  the  female 
gametangium  (Fig.  121,  a)  and  undergoes  a  period  of  rest.  Dur- 
ing this  rest  period  it  is  protected  by  the  old  gametangium  wall, 
which  remains  in  its  former  position  around  the  egg  cytoplasm. 
The  zygote  is  abundantly  furnished  with  reserve  food  in  the  form 
of  oil  and  with  the  chloroplasts  of  the  original 
unfertilized  female  gamete.  The  germination 
and  growth  of  the  zygote  cell  (6)  to  form  a  new 
gamete  plant  is  a  com- 
paratively simple  process, 
since  the  adult  gamete 
plant  is  unicellular  and  its 
growth  from  the  spherical 
zygote  cell  consists  mainly 
in  a  great  elongation  of  the 
latter,  with  accompanying 

branching  of  the  filament  thus  produced.  During  this  elongat- 
ing and  branching  process  new  nuclei  and  chloroplasts  are 
being  formed  by  repeated  divisions  of  the  chloroplasts  and  con- 
jugate nucleus  of  the  zygote.  The  resulting  gamete  plant  re- 
sembles exactly  the  original  mother  plant,  and  in  time,  after  a 
period  of  vegetative  activity,  reverts  to  reproduction. 

Asexual  reproduction.  In  Vaucheria  sessilis  another  mode  of 
reproduction  occurs  which  is  asexual  in  its  nature.  In  this 
process  the  end  of  a  filament  swells  up  and  becomes  separated 
into  a  distinct  cell  by  a  transverse  cell  wall  (Fig.  122,  a). 
The  cytoplasmic  contents  of  the  terminal  cell  then  rounds  up 
and  sends  out  minute  cytoplasmic  projections  called  cilia,  thus 
forming  a  large  motile  asexual  naked  cell,  or  zoospore  (5).  By 
the  rupture  of  the  end  of  the  enveloping  wall  this  zoospore 
becomes  a  free-swimming  cell  (<?),  which  later  comes  to  rest  and 
grows  into  a  new  plant,  clothed  with  a  cell  wall  (Fig.  122,  d,  e). 


FIG.  121.    Germination  of  the  zygote 
in  Vaucheria 

a,  zygote  still  attached  to  a  filament  ;  6,  zygote 
sending  out  a  new  filament 


THE 


233 


Zodsporangia 


Zoospore 


Life  history.  The  life  history  of  Vaucheria  thus  presents 
a  marked  contrast  to  that  of  Spirogyra  or  Protococcus.  Its 
gametangia  are  unlike  and  are 
often  for  this  reason  termed  the  an- 
theridium,  or  male  gametangium, 
and  the  oogonium,  or  female 
gametangium. 

The  female  gamete  is  a  large, 
stationary  cell  abundantly  stored 
with   nutriment   for   the   produc- 
tion of  a  new  gamete  plant.    The 
male  gametes  are  small   and  ac- 
tively motile,   and   are 
produced  in  great  num-     ^^^    Old  zoospore 
bers  to  insure  fertiliza- 
tion  of   the    nonmotile 
egg.    The  gamete  plant 
results    from    the    ger- 
mination of  the  zygote 
produced  by  the  union 
of  the  male  and  female 
gametes. 

Asexual  reproduction 
is  also  provided  for  by 
highly  specialized  motile  asexual  bodies  called  zoospores.  The 
life  history  of  Vaucheria,  therefore,  presents  the  same  stages  of 
sexual  and  asexual  reproduction  as  Chlamydomonas. 


''Holdfasts 

FIG.  122.   Asexual  reproduction  in  Vaucheria 

a,  b,  formation  and  expulsion  of  the  zoospore  c ; 
<7,  early  germination  of  the  zoospore;  e,  new  fila- 
ment formed.  «,  b,  and  c,  after  Strasburger;  d  and 
e,  after  Sachs 


(EDOGONIUM 

(Edogonium  is  one  of  the  highest  types  of  the  filamentary 
algse,  in  which  the  filaments  are  composed  of  many  cells.  The 
general  facts  regarding  its  life  history  are  similar  to  those  of 
Vaucheria  in  that  it  reproduces  by  both  the  sexual  and  the 
asexual  method.  The  female  gametangium  is  called  an  oogo- 
nium and  the  male  gametangium  an  antheridium,  as  in  Vaucheria. 
Both  organs  arise  from  ordinary  cells  of  the  filament. 


234 


GENERAL  BOTANY 


The  female  gametangium  (Fig.  123,  a)  is  a  greatly  enlarged 
cell  of  the  filament,  in  which  the  protoplast  differentiates  as  a 
large  female  gamete.  The  male  gametangia  are  developed  from 
small  cells  of  a  filament,  in  which  each  protoplast  usually  forms 
two  male  gametes.  Fertilization  results  in  a  resting  zygote, 
which,  after  a  period  of  rest,  germinates  by  the  division  of  its 
protoplast  to  form  four  free-swimming  zoospores  (Fig.  123,  £,  d). 


Ooyonhim 


Female 
fjamete 


-  Zygote  -/-f 

...Cell 
v      wall 
"Cytoplasm 
iiclcus 
Zoospores 


Antheridia 


FIG.  123.    Sexual  reproduction  in  (Edogonium 

a,  gametes  and  fertilization;  6,  zygote  with  heavy  cell  wall  in  resting  condition; 
c,  cell  division  of  the  protoplast  to  form  four  zoospores;  d,  zoospores  formed  from 
the  divisions  of  the  protoplast  in  c ;  e,  growth  of  the  zoospores  into  young  plants  after 
becoming  attached  to  a  rock  in  the  water,  a,  after  Coulter ;  b-d,  after  Him ; 

e,  after  Juranyi 

These  zoospores  become  free-swimming  by  the  disorganization 
of  the  old  zygote  cell  wall,  come  to  rest  on  a  support,  and  begin 
to  elongate  into  a  new  filament.  This  new  plantlet  (e)  is  at 
first  attached  but  later  becomes  a  free-floating  filament  like  the 
parent  organism. 

In  asexual  reproduction  zoospores  are  formed  much  as  in 
Vaucheria  except  that  they  arise  from  protoplasts  of  cells 
within  the  filament  (Fig.  124).  Each  zoospore  becomes  free 
from  the  mother  plant  by  the  rupture  of  the  mother-cell  wall 
and  swims  freely  in  the  water  for  a  short  period.  It  then  loses 
its  cilia,  becomes  attached  to  a  support,  and  forms  a  new 
filament,  exactly  as  in  the  case  of  zoospores  developed  within 
the  zygote. 


THE  ALG^E 


235 


Life  history.  The  life  history  of  (Edogonium  is  therefore  similar 
to  that  of  Chlamydomonas  and  Vaucheria  in  having  an  asexual 
phase,  represented  by  motile  zoospores,  and  a  sexual  phase  with 
gametes.  The  asexual  phase,  as  in  the  other  algee  mentioned, 
enables  the  alga  to  increase  rapidly  during  periods  adapted  to 
rapid  growth,  while  the  sexual  phase  enables  the  plant,  by  means 
of  the  highly  protected  zygote,  to  pass 
inclement  seasons  without  danger. 


FUCUS   VESICULOSUS  (BLADDER 
WRACK) 


Zoospore.. 
formation 


FIG.  124.   Asexual  reproduc- 
tion in  (Edogonium 

a,  two  zoospores  forming,  from 
the  protoplasts  of  two  cells  of 
a  filament ;  b,  free-swimming 
zoospore ;  c,  zoospore  come  to 
rest  and  beginning  to  elongate 
into  a  new  plant;  d,  young 
plant  formed  from  a  zoospore. 
After  West 


Habit.  Fucus  grows  along  the  sea- 
shore and  constitutes  a  conspicuous 
part  of  the  marine  flora  of  these  coastal 
regions.  The  color  is  due  to  a  brown 
pigment  secreted  by  the  chloroplasts, 
which  masks,  wholly  or  in  part,  the 
green  chlorophyll  pigment  which  is 
also  secreted  by  the  chloroplastids  of 
Fucus.  The  plant  body  is  admirably 
adapted  to  photosynthesis,  since  it  is 

flattened  like  a  leaf  and  forms,  at  regular  intervals,  air  cavities, 
or  floats  (Fig.  126),  which  buoy  the  plant  up  in  the  sea  water  and 
expose  its  flattened  branches  to  the  sunlight.  The  plants  elongate 
by  means  of  growing  points  at  the  branch  tips,  which  fork  in  a 
dichotomous  manner,  thus  producing  numerous  flattened  branches 
all  lying  in  one  plane.  They  are  attached  to  rocks  or  pier  posts 
along  the  ocean  shores  by  means  of  rootlike  holdfasts,  which 
serve  to  anchor  the  plant  body  but  do  not  serve  for  the  absorp- 
tion of  raw  food  materials,  since  these  foods  can  be  absorbed  by 
the  entire  epidermal  surface. 

Fucus  is  much  more  complex  in  structure  than  Spirogyra,  being 
composed  of  cells  differentiated  into  an  epidermis,  a  brown  cortex, 
and  a  central  cylinder,  or  medulla,  composed  of  elongated  con- 
ducting cells  in  the  form  of  strands  or  chains.  Fucus  is  thus  struc- 
turally adapted  to  withstand  the  buffeting  of  the  ocean  waves. 


THE 


237 


Reproduction.  In  Fucus,  as  in  Spirogyra,  reproduction  is 
accomplished  largely  by  the  sexual  process,  although  vegetative 
reproduction  may  take  place  by  the  growth  of  parts  of  the  plant 
body  which  chance  to  be  broken  off  by  the  waves  or  by  other 
means.  The  male  and  female  gametangia  are  borne  on  separate 
plants  within  distinct  male  and  female  branches  (Fig.  126). 


•Receptacle* 


•Plant  body 
Floats 


Male  conceptacle 


Ostiu 


FIG.  126.    Plant  body  and  male  reproductive  organs  of  Fucus 

a,  plant  body,  male  receptacles,  and  floats:    6,  male  conceptacle  with  antheridial 
branches ;  c,  antheridial  branch  with  antheridia 

Like  Spirogyra,  Fucus  is  therefore  dioecious,  and  the  male 
gametes  seek  the  females  in  the  open  sea  during  the  fertilization 
period.  Both  male  and  female  gametangia  are  borne  in  flask- 
shaped  cavities,  —  the  reproductive  cavities,  or  conceptacles  (5). 
Within  the  reproductive  branches  each  cavity  opens  to  the  sur- 
face by  means  of  a  narrow  neck  and  pore,  through  which  the 
gametes  are  finally  expelled  into  the  sea  water  at  the  time  of 
fertilization.  When  the  male  gametes  are  being  shed  into  the 
water  they  give  the  male  reproductive  branches  a  yellowish  color, 
while  the  eggs  impart  a  greenish  tint  to  the  female  branches. 


238 


GENERAL  BOTANY 


Conceptacle 


Both  male  and  female  gametangia  arise  from  the  cells  bound- 
ing the  walls  of  the  male  and  female  reproductive  cavities  as 
hairlike  cellular  outgrowths,  not  unlike  a  root  hair  growing  from 
an  epidermal  cell  of  a  root  tip. 

In  the  case  of  the  female  gametangia  each  initial  hairlike  game- 
tangium cell  divides  to  form  one  or  more  stalk  cells  and  a  ter- 
minal cell,  which  enlarges  to  form  the  gametangium  proper 

(Fig.  127).  This  female  gametan- 
gium in  Fucus  differs  materially 
from  that  of  the  green  algse  in 
forming  more  than  one  gamete 
in  each  gametangium.  We  shall 
therefore  expect  to  find  the  process 
of  gametogenesis  much  more  com- 
plex in  Fucus  than  in  Spirogyra, 
where  the  entire  protoplast  of  an 
ordinary  cell  simply  contracts  and 
rounds  up  to  form  one  gamete.  In 
the  male  cavity  each  initial  hair- 
like  cell  divides  into  a  cell  chain 
and  branches  repeatedly,  forming 
a  shrublike  system  of  branched 
filaments.  The  end  cells  of  these 
branches  develop  the  gametangia, 
in  which  numerous  male  gametes 
are  formed  during  gametogenesis. 
Cramete  formation  takes  place  by  similar  processes  in  both 
male  and  female  gametangia,  although  the  gametes  which  result 
are  very  different,  both  in  number  and  in  size,  in  the  two  cases. 
Three  essentially  distinct  processes  are  involved  in  this  forma- 
tion of  the  male  and  female  gametes,  namely,  nuclear  division, 
cell  division,  and  gamete  differentiation  (Fig.  128,  a-c). 

Nuclear  division  takes  place  first  in  each  gametangium,  forming 
eight  free  nuclei  in  the  female  gametangium  and  sixty-four  in  the 
male  gametangium.  These  nuclei  soon  become  uniformly  distrib- 
uted throughout  the  cytoplasm  of  each  large  gametangium  cell, 
and  then  an  internal  process  of  cell  division  follows  which  results 


Paraphysis 


FIG.  127.    Female  receptacle  and 
conceptacles  of  Fucus 

a,  female  receptacle  and  ostia;    b, 

transverse  section  of  a,  showing. con- 

ceptacles;     c,  enlarged  conceptacle, 

oogonia,  and  paraphyses 


THE  ALG.E 


239 


'Eggs 


in  the  separation  of  the  mother  cells  of  the  future  gametes  by 
membranous  walls.  In  this  internal  cell  division  a  cleavage  of  the 
cytoplasm  around  each  nucleus  is  followed  by  the  simultaneous 
secretion  of  the  membranous  walls  between  the  mother  cells. 
The  protoplast  of  each  mother  cell  then  rounds  up  in  the  female 
gametangium  (Fig.  128,  b  and  c),  the  dividing  membranous  walls 
are  absorbed,  and  the  gametes 
are  ready  to  be  expelled  from 
the  gametangium.  In  the  male 
gametangium  (Fig.  129)  simi- 
lar processes  occur,  except  that 
the  male  gametes  differentiate 
to  form  two  laterally  attached 
cilia  and  a  brick-red  eyespot  in 
a  more  or  less  pear-shaped  male 
gamete.  When  the  gametes  are 
fully  formed,  they  are  expelled 
into  the  sea  water  through 
the  pore  of  each  reproductive 
cavity  embedded  in  a  slimy 
substance.  This  expulsion  of 
gametes  usually  occurs  at  low 
tide,  when  the  reproductive 
branches  dry  and  contract  in 
the  sun,  thus  squeezing  out 
the  contents  of  the  reproduc- 
tive cavities.  When  first  ex- 
pelled, the  male  and  female  gametes  are  both  inclosed  in  the 
inner  membranous  layer  of  the  gametangium  wall,  but  this  soon 
ruptures  and  the  individual  gametes  become  free-floating  cells 
(Figs.  128,  d,  and  129,  5).  With  the  return  of  the  tide  the 
gametes  from  the  male  plants  are  mixed  freely  with  the  female 
gametes  in  the  sea  water,  and  abundant  fertilizations  occur,  as 
is  evidenced  by  the  great  number  of  these  plants  in  locations 
favorable  to  their  existence. 

In    the    process    of   fertilization    the    female    gametes    float 
impassively  in  the  water,  but  the  male  gametes  swarm  in  great 


FIG.  128.  Development  of  the  oogonium 
and  eggs  in  Fucus 

a,  young  oogonial  cell  with  eight  free 
nuclei;  &,  older  oogonium  divided  to 
form  eight  eggs ;  c,  eggs  in  the  oogonium 
rounded  up  preparatory  to  expulsion  as 
in  d.  After  Thuret 


240 


GENERAL  BOTANY 


numbers  around  them,  probably  attracted  by  a  secretion  from 
the  female  gamete.  When  one  male  gamete  has  succeeded  in 
penetrating  the  outer  cytoplasmic  layer  of  the  female  gamete, 
the  other  swarming  male  gametes  either  disappear  or  die  in  con- 
tact with  the  egg.  Cytoplasmic  and  nuclear  fusions  complete 

the  act  of  fertilization,  and  the 
fertilized  egg  becomes  the  zy- 
gote, which  quickly  secretes 
a  cellulose  wall  and  begins  to 
divide  to  form  a  new  Fucus 
offspring  (Fig.  129  d,  e). 

Germination  of  the  zygote 
follows  immediately  upon  the 
fertilizing  act  instead  of  after 
a  long  rest  period  as  in  Spi- 
rogyra  and  most  fresh-water 
algae.  This  early  germination 
of  the  zygote  is  undoubtedly 
connected  with  the  fact  that 
Fucus  lives  in  the  sea,  where 
the  danger  of  extermination  in 
winter  or  in  dry  periods  is 
much  less  than  in  fresh-water 


FIG.  129.    Development  of  the  male 
gametes,   fertilization,   and    develop- 
ment of  the  embryo  in  Fucus 


a,  antheridium  with  sperm  mother  cells ;  ponds    and     lakes.      Winter   is 

6,  expulsion  of  free  sperms ;  c.  swarming  , -,  ^  /.  n          .         ,  -, 

of  sperms  around   an  egg  preparatory  thus     Passed      safely      m      the 

to  fertilization  ;  d,  the  cellular  embryo ;  vegetative    condition,    and    the 

e,   more  advanced    embryo   of  Fucus.  i  •    11  ,     i         ,• 

After  Thuret  highly  protected  resting  zygote 

is  not  a  necessity  in  its  life 

cycle.  Germination  of  the  zygote  results  in  a  many-celled 
embryo,  which  differentiates  to  form  a  growing  point  and  a 
holdfast  by  which  the  young  offspring  becomes  anchored  to  a 
stone  or  other  object.  Elongation  and  repeated  forking  of  the 
original  growing  point  follows,  which  finally  results  in  a  new 
offspring  like  the  parent. 

Life  history.  The  life  history  of  Fucus  does  not  differ  materially 
from  that  of  Spirogyra  except  in  the  fact  that  no  resting  zygote 
is  formed  to  tide  over  inclement  periods.  Asexual  reproduction  is 


THE  ALG.E  241 

also  very  limited  in  Fucm,  as  it  may  well  be  on  account  of  the 
abundant  production  of  male  and  female  gametes  and  the  greater 
security  of  the  offspring  in  the  more  permanent  and  less  variable 
habitat  in  the  ocean.  The  graphical  history  of  a  Fucus  plant  will 
therefore  differ  very  little  from  that  of  Spirogyra  except  in  the 
details  of  structure  and  in  processes  concerned  with  reproduction. 
On  account  of  nuclear  phenomena  connected  with  the  forma- 
tion of  the  gametes  in  Fucus  the  plant  body  cannot  properly  be 
called  a  gametophyte.  The  details  of  this  matter  do  not,  however, 
belong  in  an  elementary  course  in  biology  and  are  therefore 
passed  over  with  this  brief  note. 


CHAPTER  XIII 

THE  FUNGI 

Mode  of  Life.  The  fungi  differ  from  all  the  plants  thus  far 
studied  in  that  they  lack  the  green  pigment  chlorophyll,  which 
enables  the  algae  and  the  higher  spore  and  seed  plants  to  manu- 
facture their  own  organic  foods  from  carbon  dioxide,  water,  and 
salts  derived  from  the  soil.  The  fungi  are  therefore  dependent 
plants,  living  either  as  saprophytes,  upon  lifeless  organic  matter, 
or  as  parasites,  upon  living  animals  and  plants. 

Saprophytes.  The  greater  number  of  fungi,  including  the 
familiar  mushrooms,  molds,  and  yeasts,  are  saprophytes,  which 
feed  upon  and  consume  lifeless  organic  bodies  or  their  products. 
Such  fungi  serve  a  useful  purpose  as  scavengers,  since  they  tend 
to  dispose  of  the  dead  remains  of  plants  and  animals,  which 
might  otherwise  hinder  the  growth  or  imperil  the  life  of  other 
living  organisms.  This  is  especially  evident  in  a  forest,  where 
saprophytic  mushrooms,  leaf  molds,  and  bacteria  convert  the 
annual  fall  of  leaves,  tree  trunks,  and  branches  into  the  ingredi- 
ents of  leaf  mold,  which  is  a  characteristic  component  of  the 
forest  soil.  The  accumulation  of  this  annual  leaf  fall  would 
be  disastrous  to  the  living  trees  and  plants  of  the  forest  if  it 
did  not  constantly  disappear  by  decay  through  the  agency  of 
saprophytic  fungi. 

Parasites.  The  parasitic  fungi  are  less  numerous  than  the 
saprophytes.  They  include  such  forms  as  the  bacteria  of  disease, 
the  parasitic  rusts  and  smuts  of  cereal  grains,  and  the  tree- 
killing  fungi.  Such  parasites  cause  immense  damage  annually 
through  the  death  of  animals  and  the  destruction  of  valuable 
crops  and  timber  upon  which  they  prey. 

Classification.  The  fungi  have  undoubtedly  sprung  from  the 
algae  by  the  loss  of  chlorophyll  and  chloroplasts,  since  they  so 

242 


THE  FUNGI  243 

closely  resemble  these  plants  in  their  structure  and  reproduc- 
tion. They  have  nevertheless  become  profoundly  modified  by 
their  parasitic  and  saprophytic  habits  and  are  therefore  usually 
classified  into  plant  groups  distinct  from  the  green  algse. 

The  following  simple  classification  will  serve  to  illustrate 
something  of  the  great  variation  in  form,  structure,  and  repro- 
duction represented  among  the  fungi. 

1.  Bacteria,  including  the  simplest  of  the  fungi,  which  occur 
in  the  form  of  single  cells  or  in  loose  colonies  (Fig.  134). 

2.  Algal  fungi,  including  the  common  black  molds  (Figs.  139 
and  140)  and  water  molds  infesting  dead  fish  and  flies  in  ponds, 
lakes,  and   streams.     These   fungi  resemble  closely  the  green 
algse,   from  which   they  have  sprung,   in  both    structure   and 
reproduction. 

3.  Sac  fungi,  including  the  yeasts,  the  common  blue  and  green 
molds  (Figs.  143-145),  and  the  blights  found  on  the  leaves  of 
trees  and  various  plants.    These,  with  the  exception  of  the  yeasts, 
are  moldlike  fungi,  which  form  spores  in   large,  saclike  cells 
called  asci.    These  spore  sacs,  or  asci,  are  usually  inclosed  in 
characteristic  fruit  bodies. 

4.  Higher  fungi,  including  mushrooms,  puffballs,  rusts,  and 
smuts  of  cereal  grains  (Figs.  145-155).   These  higher  fungi  are 
characterized  by  the  production  of  conspicuous  fruit  bodies  for 
the  production  and  scattering  of  asexual  spores. 

YEAST 

The  origin  and  relationship  of  the  yeast  plants  to  the  other 
fungi  is  not  definitely  known,  although  they  are  usually  classi- 
fied with  the  sac  fungi.  They  are  for  the  most  part  single-celled 
plants  resembling  spores  and  are  widely  distributed  in  nature, 
since  they  reproduce  rapidly  and  are  easily  blown  about  in  the 
open  by  the  wind  or  by  currents  of  air  in  dwellings.  They  there- 
fore occur  on  almost  all  exposed  surfaces,  particularly  on  those 
which  are  moist  and  contain  sugar  or  acids,  so  that  grapes, 
apples,  and  other  fruits,  when  exposed  to  the  air,  are  sure  to  have 
some  of  these  little  plants  lodged  on  the  skin. 


244 


GENEPx  AL  BOTANY 


When  the  juice  of  such  fruits  is  pressed  out,  as  in  the  making 
of  cider,  grape  juice,  or  homemade  wines,  the  wild  yeast  plants 
are  certain  to  get  into  the  expressed  juice  and  cause  the  fermen- 
tation which  is  so  common  under  such  circumstances.  Sweet 
solutions  of  all  kinds,  when  exposed  to  the  air,  become  infected 
with  wild  yeasts,  which  causes  them  to  ferment. 

The  cultivated  yeasts  used  in  the  making  of  yeast  cakes  and 
in  the  brewing  industries  are  simply  selected  varieties  of  wild 

yeasts     which    have 

been  found  especially 
adapted  to  the  rais- 
ing of  bread  or  to 
the  making  of  various 
sorts  of  fermented 
liquors.  These  culti- 
vated yeasts  are  also 
of  many  kinds,  each 
particular  kind  hav- 
ing its  peculiar  char- 
acteristic effect  on 
sugary  solutions  in 
which  they  are  placed. 
Thus,  in  making  beer 


& 


FIG.  130.    Various  kinds  of  wine  and  beer  yeasts 


a,  6,  wine  yeast  (S.  ellipsoideus)  (a,  young  and  vigo-      ^he  people  of  some  of 

rous;  6,  old  (1)  and  dead  (2)) ;  c,  d,  beer  yeast  (S.  cere- 

visite)  (c,  bottom  yeast;  <7,  top  yeast).  After  Marshall      the    European    COU11- 

tries  use  what  is  called 

bottom  yeast  (Fig.  130),  which  is  capable  of  producing  only  a 
moderate  supply  of  alcohol  by  fermentation.  Top  yeast,  which 
is  used  in  making  English  beers  and  compressed  yeast  cakes, 
produces  a  larger  amount  of  alcohol  than  bottom  yeast.  Dis- 
tillers' yeasts,  used  in  making  rum  and  brandy,  are  also  capable 
of  producing  large  quantities  of  alcohol  by  fermentation.  The. 
various  kinds  of  wines  likewise  owe  their  peculiar  flavors  to  the 
kind  of  yeast  used  in  them.  All  these  cultivated  yeasts  have  been 
gradually  selected  by  man  for  particular  uses,  just  as  various 
kinds  of  fruit  trees,  cereals,  and  flowers  have  been  selected 
and  improved  by  cultivation. 


THE  FUNGI 


245 


Structure.  The  yeasts  are  composed  of  minute  spherical  or 
slightly  elongated  cells  varying  from  ^ ^ o"  to  loTTo  °^  an  mc^ 
in  diameter.  They  usually  occur  as  single  cells,  but  may  adhere 
to  form  cell  chains  or  loose  temporary  colonies.  The  yeast  cell 
(Fig.  131)  has  the  same  general  structure  as  that  of  plant  cells 
with  which  we  are  familiar.  The  cell  wall  is  a  thin  membrane 
composed  of  two  or  more  layers,  and  the  cytoplasm  is  filled  with 
granules  of  various  sizes, 
some  of  which  are  sup- 
posed to  be  composed  of 
fatty  material,  probably 
useful  as  a  reserve  food. 
The  nucleus  can  only 
be  observed  in  material 
which  has  been  specially 
stained  for  that  purpose, 
and  is  found  in  close 
proximity  to  the  large 
central  vacuole  which 
fills  the  greater  part  of 
the  cavity  of  the  mature 
yeast  cells. 

Reproduction.  If  yeast 
plants  are  placed  in  a 
sugary  solution,  such  as 
beer  wort,  at  a  temperature  of  from  25°  to  30°  centigrade,  they 
begin  to  reproduce  new  plants  by  the  process  known  as  bud- 
ding. Budding  consists  in  an  outgrowth,  or  protrusion,  of  the 
cell  wall  of  the  yeast  cell  into  which  the  cytoplasm  flows,  thus 
forming  a  new  cell.  The  outgrowth  is  very  minute  at  first,  but 
by  growth  it  finally  reaches  the  size  of  the  mother  cell  from 
which  it  grew.  Finally  a  cell  wall  is  formed  between  the  mother 
cell  and  the  bud,  and  the  two  cells  split  apart,  thus  forming  two 
new  yeast  plants  (Fig.  131,  d  and  e).  Division  of  the  nucleus 
accompanies  the  budding  process  in  such  a  manner  that  each  new 
bud  which  is  formed  into  a  new  yeast  plant  is  furnished  with  a 
nucleus.  Since  the  budding  process  goes  on  with  extreme  rapidity 


FIG.  131.    Yeast  cells  highly  magnified 

a  and  b,  showing  vacuoles  and  granular  cyto- 
plasm; c,  showing  nucleus;  d,  budding  yeast  cell 
with  dividing  nucleus;  e,  the  bud  cut  off  from 
the  mother  cell  with  its  portion  of  the  nucleus. 
After  Conn 


246 


GENERAL  BOTANY 


at  the  proper  temperature,  new  plants  are  formed  in  almost 
incredible  numbers  in  a  comparatively  short  time  when  yeast  is 
placed  in  a  favorable  medium,  such  as  bread  sponge  or  beer  wort. 
When  young,  vigorous  yeast  cells  are  grown  at  the  proper  tem- 
perature and  moisture  but  are  lacking  in  an  adequate  food  supply, 
they  sometimes  form  internal  spores  (Fig.  133)  by  division  of  the 
nucleus  and  aggregation  of  the  cell  cytoplasm  around  each  new 
nucleus.  From  4  to  8  spores  are  usually  formed  in  each  cell, 
each  spore  consisting  of  cytoplasm  and  nucleus  surrounded  by  a 
cell  wall.  Finally  the  mother-cell  wall  breaks  down,  and  the  lib- 
erated spores  form  new 
yeast  plants.  The  proc- 
ess of  spore  formation 
in  yeast  is  apparently 
designed  to  preserve  the 
yeast  during  inclement 
conditions,  rather  than 
as  a  method  for  more 
rapid  reproduction  or 

The  figure  shows  how  a  cell  group  (d)  may  be      dissemination, 
formed  in  actively  growing  yeast  if  successively 

formed  buds  remain  attached.   After  Conn  Fermentation.       The 

importance  and  interest 

of  yeasts  for  man  is  due  largely  to  their  power  of  causing  alco- 
holic fermentations  in  sugar  solutions,  since  it  is  upon  this  power 
that  the  modern  baking  and  brewing  industries  are  based. 

If  yeast  plants,  in  the  form  of  commercial  yeast  cakes  or  in 
a  free  state,  are  placed  in  a  sugar  solution  under  the  proper 
temperature  (25°-30°  C.),  bubbles  of  carbon-dioxide  gas  will 
soon  rise  to  the  surface  of  the  liquid,  indicating  that  the  process 
of  fermentation  has  begun.  Proper  chemical  tests  will  also 
reveal  the  presence  of  alcohol,  which  is  formed  in  the  fermenting 
solution  at  the  same  time  as  the  carbon  dioxide.  It  has  been 
found  that  the  carbon  dioxide  and  alcohol  are  produced  by 
the  splitting  up  of  the  sugar  molecule  by  the  yeast  ferment 
called  zymase. 

The  carbon  dioxide  which  rises  to  the  surface  is  utilized  in 
the  process  of  bread-making  to  make  the  bread  light,  while  the 


FIG.  132.    Growing  yeast  with  buds  and 
cell  chains 


THE  FUNGI 


247 


alcohol  is  retained  in  the  commercial  manufacture  of  beer,  wine, 
and  all  fermented  liquors.  The  splitting  of  the  sugar  by  the 
ferment  is  represented  by  the  following  equation: 

Grape  sugar  -f  zymase  =  alcohol  +  carbon  dioxide  +  zymase. 

The  above  fermentation  process  was  for  a  long  time  thought 
to  be  due  to  the  activity  of  the  protoplast  of  the  living  yeast 
cells,  which  were  therefore  called  living  ferments.  It  remained 
for  Buchner,  a  great  German  scientist,  to  demonstrate  that  the 
active  body  in  the  process  of  fermentation  is  not  the  protoplast 
of  the  yeast  cell  but  rather  a  product  of  the  cell  protoplasm  in 
the  form  of  a  secretion 
or  fermenting  substance, 
which  is  now  termed  zy- 
mase. Buchner  ground 
yeast  cells  in  fine  infu- 
sorial earth  and  filtered 
the  juice  pressed  from 
the  yeast  cells  through 
a  porcelain  filter.  The 
filtered  extract  thus  ob- 
tained caused  alcoholic 
fermentation  in  sugar 
solutions  without  the  presence  of  living  yeast  cells.  As  a  result 
of  this  experiment  Buchner  concluded  that  an  active  ferment 
was  secreted  by  the  yeast  cell,  which  passed  through  his  filters 
and  caused  fermentation  in  the  sugar  solutions  to  which  it  was 
added.  This  ferment  is  not  liberated  from  the  yeast  cell  in 
normal  fermentation,  but  the  sugar  is  first  absorbed  into  the 
cell  and  is  then  attacked  by  the  zymase.  Such  a  ferment,  or 
enzyme,  which  acts  within  the  cell  which  secretes  it,  is  called 
an  endoenzyme  or  ferment.  In  many  other  instances  living  cells 
form  exoenzymes,  which  are  first  liberated  into  the  surrounding 
medium  before  they  cause  active  fermentation. 

The  process  of  fermentation  in  yeasts  is  not  clearly  understood. 
A  distinctive  feature  of  the  process  is  that  the  ferment  is  neither 
diminished  in  amount  nor  injured  for  further  use  by  its  activity. 


FIG.  133.    Spore  formation  in  yeast 
After  Conn 


248  GENERAL  BOTANY 

At  the  end  of  a  fermenting  process,  therefore,  the  same  amount 
of  ferment  is  left  over  as  there  was  present  at  the  beginning. 
This  is  indicated  in  the  above  equation  for  the  fermentation  of 
grape  sugar,  in  which  the  zymase  appears  on  both  sides  of  the 
equation.  It  is  estimated  that  zymase  can  convert  as  much  as 
100,000  times  its  own  volume  of  sugar  into  alcohol  and  carbon 
dioxide  without  being  diminished  in  volume  or  in  the  power  to 
cause  renewed  fermentation  in  a  sugary  solution.  Other  fer- 
ments behave  similarly,  -acting  like  the  so-called  catalytic  agents 
among  inorganic  substances. 

Digestion  has  already  been  defined  as  the  transformation  of 
solid  foods  into  soluble  food  substances  which  can  diffuse  through 
cell  membranes  and  be  used  in  the  making  of  new  living  sub- 
stance. Some  cases  are  known,  however,  in  which  soluble  sub- 
stances need  to  be  changed  chemically  before  they  are  of  use 
for  assimilation  by  plant  and  animal  cells.  This  appears  to  be 
the  case  with  cane  sugar,  which  is  first  changed  to  grape  sugar 
in  plants  and  is  then  capable  of  being  used  in  assimilation. 

If  yeast  cells  are  placed  in  cane-sugar  solutions,  they  secrete 
cane-sugar  ferments  (maltase  or  invertase),  which  change  the  cane 
into  a  grape  sugar.  Alcoholic  fermentation  yields  energy  which  is 
available  to  the  yeast  cell,  while  digestive  fermentation  prepares 
a  food  for  assimilation  by  the  cell  protoplasm.  In  both  cases  the 
active  agent  in  the  chemical  reactions  involved  is  a  secretion  of 
the  yeast  cell,  which  is  called  an  enzyme  or  a  ferment. 

Cane  sugar  +  water  -f-  cane-sugar  ferment 

=  grape  sugar  +  cane-sugar  ferment. 

Grape  sugar  +  zymase  ferment 

=  alcohol  +  carbon  dioxide  -f-  zymase. 

Ferments  may  be  classified,  therefore,  into  energy -forming  and 
digestive  ferments,  according  to  the  results  of  their  activity. 

All  cases  of  digestion  in  animals  and  plants  are  caused  by  the 
presence  of  active  ferments,  which  are  secretions  of  plant  or 
animal  cells.  In  the  higher  animals  these  secreting  cells  usually 
occur  in  the  form  of  glands,  but  in  higher  plants  they  may  be 


THE  FUNGI  249 

ordinary  leaf  cells,  wood-ray  cells,  or  root  cells.  Among  the 
higher  fungi,  as  we  shall  learn  later,  a  great  variety  of  digestive 
and  energy -forming  ferments  are  secreted,  which  aid  both  para- 
sitic and  saprophytic  fungi  in  securing  their  food. 

The  use  of  fermentation  to  the  yeast  plant,  beyond  the  diges- 
tion of  the  small  amount  of  food  needed  for  reproduction  and 
growth,  is  not  certainly  known.  It  is  supposed  that  it  uses 
some  of  the  energy  produced  by  fermentation  for  its  life  proc- 
esses, as  the  higher  plants  and  animals  use  the  energy  resulting 
from  respiration.  This  idea  seems  probable,  for  the  reason  that 
certain  yeasts  can  live  and  grow  for  a  considerable  time  in  a 
sugar  solution  without  the  presence  of  oxygen  for  normal  res- 
piration. Other  organisms,  namely,  bacteria  and  certain  molds, 
are  able  to  produce  energy  by  fermentation  in  the  absence  of 
oxygen.  This  has  led  to  the  classification  of  organisms  as 
aerobic  and  anaerobic,  according  to  their  method  of  securing 
energy  for  the  life  processes. 

Aerobic  organisms  secure  this  energy  by  means  of  oxygen 
used  in  normal  respiration,  while  anaerobic  organisms  are  able 
to  secure  it  by  fermentations  in  the  absence  of  oxygen.  Some  of 
the  yeasts  are  able  to  accommodate  themselves  to  either  mode  of 
life,  and  are  thus  either  aerobic  or  anaerobic,  according  to  the 
nature  of  the  medium  in  which  they  are  placed. 

Bread  making.  In  the  making  of  bread  some  active  yeast, 
composed  of  starch  mixed  with  some  sugar  and  water,  is  added 
to  bread  sponge.  The  action  of  the  yeast  on  the  sponge  is  a 
double  one,  caused  by  the  secretion  of  a  digestive  cane-sugar 
ferment  and  of  an  alcoholic  energy-forming  ferment,  zymase. 
The  cane-sugar  ferment  transforms  the  cane  into  grape  sugar, 
as  explained  above,  while  the  zymase  completes  the  transforma- 
tion of  grape  sugar  into  alcohol  and  carbon  dioxide.  The  carbon 
dioxide  is  liberated  in  the  sponge  and  causes  the  rising  of  the 
bread.  The  almost  imperceptible  amount  of  alcohol  produced 
disappears  in  the  baking.  The  two  formulae  given  above  under 
digestion  would  therefore  indicate  in  general  terms  the  action 
of  the  yeast  ferments  in  bread  making.  The  cane  sugar  upon 
which  the  yeast  acts  must  be  supplied  in  the  sponge. 


250  GENEBAL  BOTANY 

Beer  and  wine  making.  In  the  making  of  beer  and  wine  the 
ferments  and  the  processes  involved  are  identical  with  those 
just  described  for  bread  making.  The  difference  in  the  final 
practical  result  is  that  in  beer  and  wine  making  the  alcohol  is 
saved  and  the  carbon  dioxide  is  largely  allowed  to  escape. 
The  sugar  for  the  fermentative  process  may  be  supplied  directly 
or  it  may  be  obtained  from  sprouted  barley  (called  malt),  from 
expressed  grape  juice,  or  from  the  juices  of  other  fruits.  The 
special  flavors  of  different  wines  and  beers  are  due  in  part  to 
the  kind  of  yeast  plants  used  and  in  part  to  the  source  from 
which  the  sugar  solution  is  derived. 

In  the  preparation  of  malt  from  barley  the  grain  is  first 
sprouted,  in  order  that  the  stored  starch  of  the  barley  seed 
may  be  digested  and  so  transformed  into  a  cane  sugar.  This 
digestion  is  due  to  the  secretion  of  digestive  ferments  by  the 
storage  cells  of  the  seeds.  The  cane  sugar  (maltose)  thus  formed 
is  dissolved  out  in  water  from  the  dried  and  crushed  seeds,  and 
furnishes  the  sugar  for  the  action  of  the  beer  yeasts.  These  yeasts 
then  transform  the  cane  sugar  into  grape  sugar,  and  this  again 
into  alcohol  and  carbon  dioxide,  exactly  as  described  above  in 
bread  making. 

The  great  value  of  the  yeasts  in  the  industries  is  due  to  the 
rapid  reproduction  of  the  yeast  cells  by  budding,  and  to  the 
peculiar  nature  of  the  secreted  ferment,  which  is  able  to  trans- 
form large  volumes  of  sugar  into  alcohol  and  carbon  dioxide 
without  losing  its  active  properties. 

BACTERIA 

Bacteria  are  plants  of  the  greatest  interest  and  importance 
to  man  on  account  of  their  relation  to  disease,  to  the  destruc- 
tion of  food,  and  to  the  maintenance  of  certain  modern  indus- 
tries. It  is  not  generally  known  that  bacteria,  as  well  as  yeasts, 
are  plants  which  belong  to  the  great  group  of  fungi  which  we 
are  now  studying.  Botanists  classify  them  as  fission  fungi,  or 
Schizomycetes,  on  account  of  their  mode  of  multiplying  by  simple 
cell  division,  or  fission.  They  are  derived,  like  the  yeasts,  from 


THE  FUNGI 


251 


certain  of  the  lower  groups  of  fungi,  although  neither  the  rela- 
tionship of  the  bacteria  nor  their  classification  into  definite 
groups  is  as  yet  understood  even  by  specialists  in  bacteriology. 
Bacteria  are  usually  grouped,  for  convenience,  under  certain 
form  types,  illustrated  in  Fig.  134. 

Structure.    The  cells  of  the  bacteria  are  the  smallest  of  which 
we  have  any  definite  knowledge,  ranging  from  ^Q^  to  -g^^-Q  of 


Fig.  134.   Form  types  of  bacteria 

a,  types  of  bacilli ;  6,  types  of  micrococci ;  c,  types  of  spirilla.  After  Williams.  Redrawn 
from  Marshall's  "Microbiology" 

an  inch  in  diameter.  The  rod  bacteria,  represented  in  Fig.  134, 
a  and  c,  vary  from  ^^-fr  to  g-Q-J--^^  of  an  inch  in  length. 

The  bacterial  cell  differs  somewhat  from  the  ordinary  plant 
cell  in  the  structure  of  the  cell  wall,  in  the  chemical  nature  of 
its  protoplasm,  and  in  the  nature  of  the  nucleus.  The  cell  wall 
is  composed  of  two  membranes,  the  outer  of  which  may  be- 
come gelatinous,  thus  enabling  the  cells  to  adhere  and  form 
simple  colonies. 

The  protoplasm  is  dense  and  contains  more  nitrogen  than 
that  of  ordinary  plant  cells.  The  nucleus,  if  present,  has  no 
nuclear  membrane,  and  some  bacteriologists  deny  the  presence 
of  a  nucleus  in  the  cells  of  bacteria,  while  others  claim  to  have 


252  GENERAL  BOTANY 

found  the  nucleus  in  division,  with  spindles  resembling  those 
of  the  higher  plants.  For  the  general  student  the  most  impor- 
tant thing  to  realize  is  that  bacteria  are  unicellular  organisms 
with  a  cell  structure  essentially  like  that  of  yeasts  and  other 
simple  plants.  This  being  the  case,  we  shall  expect  to  find  that 
the  life  processes  and  activities  of  bacteria  are  quite  similar  to  the 
physiological  processes  of  other  fungi. 

Many  bacteria  have  the  power  of  movement  in  the  liquids 
in  which  they  are  developed.  These  movements,  when  actually 
progressive,  are  due  to  minute,  hairlike  protrusions  of  the  pro- 
toplasm, termed  cilia  or  flagella,  which  propel  the  organisms 


OGDGD 


FIG.  135.   Diagram  showing  method  of  division  in  bacterial  cells 

a,  division  of  a  rod  bacterium  (Bacterium  or  Bacillus) ;    6,  division  of  a  cell 

of  a  coccus  or  of  a  micrococcus  form  of  bacteria.    After  Novy.    Redrawn 

from  Marshall's  "  Microbiology  " 

by  simple  or  complex  whiplashlike  movements.  The  cilia  are 
variously  distributed,  being  usually  either  terminal  or  uniformly 
distributed  over  the  cell  surface.  The  vibratory  movements  of 
bacteria  often  observed  under  the  microscope  are  not  due  to  vital 
activity  of  cilia  or  flagella,  but  resemble  the  so-called  Brownian 
movements  characteristic  of  fine  inorganic  particles  of  various 
kinds  in  liquids. 

Reproduction.  The  bacteria  reproduce  either  by  simple  division 
and  separation  of  the  two  daughter  cells  or  by  spore  formation. 
The  former  process  is  very  rapid  and  is  the  usual  method  of 
reproduction  ;  this  accounts  for  the  ability  of  bacteria  to  multiply 
with  such  startling  rapidity  as  is  often  witnessed  in  the  case  of 
diseases  and  in  decaying  or  putrefying  organic  material. 

When  a  bacterial  cell  is  about  to  divide  (Fig.  135),  the  cyto- 
plasm constricts  in  the  middle  of  the  cell  from  the  cell  wall 


THE  FUNGI  253 

inward.  This  constriction  may  take  place  in  any  axis  in  the 
spherical  bacteria,  but  in  the  rod  forms  it  always  occurs  across 
the  long  axis  of  the  cell.  The  divided  protoplast  then  secretes 
a  cell  wall  between  its  two  halves,  the  new  wall  splits,  and  the 
two  cells  thus  formed  separate  to  form  two  new  plants.  In  a  short 
time  the  two  daughter  plants  thus  formed  will  reach  maturity 
and  divide  again.  Unless  this  division  process  is  stopped  by  lack 
of  food  or  by  other  unfavorable  conditions,  the  rapid  multiplication 
mentioned  above  will  result  in  thousands  of  new  plants  in  a  few 
hours.  It  has  been  estimated 
that  under  the  most  favorable 
conditions  for  growth  and  divi- 
sion seventeen  million  bacteria 
might  be  produced  in  twenty- 
four  hours  from  one  bacterial 

cell.    This   maximum  rate    of 

TIG.  136.    Spore  formation  in  bacteria 

division    under    such    circum- 

a,  two  bacterial  cells ;  6,  granules  collect- 
StanceS    IS    Said    to    be    at    the      i,lg  to  form  spores;    c,  black  spores  in 

rate      of     One      division     every      two  ce"s-    After  Fisher     Redrawn  from 

J  Marshall's  "  Microbiology  " 

twenty  or  thirty  minutes ;  but 

such  a  maximum,  it  is  safe  to  say,  is  never  realized  in  nature 
for  any  considerable  length  of  time,  or  the  world  would  be. over- 
run with  bacteria. 

Spore  formation  in  bacteria  (Fig.  136)  takes  place  within  a 
bacterial  cell  which  acts  as  a  spore  mother  cell.  One  spore 
only  is  formed  in  each  cell,  and  the  process  begins  by  the 
collection  and  rounding  up  of  the  protoplast  in  the  middle,  or 
at  the  end,  of  the  mother  cell,  which  often  bulges  at  this  point 
with  the  accumulation  of  the  protoplast.  A  new  cell  wall  is  then 
formed  around  the  protoplast  within  the  old  mother-cell  wall, 
and  the  spore  is  complete,  although  still  within  the  cavity  of 
the  mother  cell.  These  spores  remain  in  a  resting  condition 
until  favorable  conditions  for  growth  arise,  when  they  germinate 
to  form  a  new  plant.  In  this  process  the  wall  of  the  mother  cell 
is  split  and  the  spore  wall  and  protoplast  protrude  and  expand 
into  the  form  of  the  bacterium  from  which  the  spore  sprang 
(Fig.  137).  In  the  case  of  the  rod  bacteria  the  splitting  of  the 


254  GENERAL  BOTANY 

mother  cell  takes  place  either  at  the  end  or  in  the  middle  of  the 
cell.  As  indicated  above,  one  of  the  principal  functions  of  the 
spore  is  to  carry  bacteria  over  periods  which  endanger  the  life 
of  the  ordinary  vegetative  bacterial  cells. 

Sterilization.  On  account  of  the  great  resistance  of  the  bac- 
terial spores  to  extremes  of  temperature  and  drought  the  greatest 
difficulty  is  experienced  in  destroying  them  in  food  or  in  cultures. 
One  method  of  killing  the  spores  of  bacteria  is  to  subject  the 
material  to  be  sterilized  to  flowing  steam  at  100°  C.  for  a  short 
period  on  three  successive  days.  This  is  the  discontinuous 
method  of  sterilization  advanced  by  Tyndall.  If  milk  or  any 


o  o  0  0  0  0      ....  Odl    (X    • 

FIG.  137.   Different  methods  of  spore  germination 

a,  "direct  conversion  of  a  spore  into  a  bacillus  without  the  shedding  of  the  spore 

wall "  in  B.  leptosporus ;  b,  polar  germination  of  a  spore  in  B.  anthracis ;  c,  equatorial 

spore  germination  in  B.  subtilis;  d,  equatorial  germination  in  B.  megatherium-,  et 

same  with  horseshoe  appearance.  After  Novy.   From  Marshall's  "  Microbiology  " 

other  material  is  to  be  freed  from  bacterial  spores  by  this  method, 
it  is  first  heated  in  flowing  steam  at  100°  C.  for  fifteen  minutes, 
which  is  sufficient  to  kill  all  living  vegetative  yeast,  mold,  or 
bacterial  cells  in  the  substance,  except  certain  resistant  bacterial 
spores.  Any  spores  which  may  remain  will  germinate  if  left  for 
a  few  hours  at  the  proper  temperature.  The  material  to  be 
sterilized  is  consequently  allowed  to  stand  after  the  first  heating 
until  any  spores  which  remained  have  had  time  to  germinate 
and  form  vegetative  bacteria,  when  a  second  heating  will  kill 
all  of  the  bacterial  cells  thus  produced.  A  third  heating  is 
usually  necessary  after  an  interval,  in  order  to  kill  any  bacteria 
which  may  have  been  produced  by  spores  which  had  not  yet 
germinated  at  the  time  of  the  second  heating.  It  is  now  thought 
that  the  highly  resistant  spores  are  also  gradually  weakened 
and  finally  killed  during  the  prolonged  period  of  heating. 


THE  FUNGI  255 

Another  common  method  of  sterilization  is  to  subject  the 
material  to  be  sterilized  to  steam  at  a  very  much  higher  temper- 
ature (120°  C.)  at  the  outset  than  that  indicated  above.  Then 
a  single  heating  is  usually  sufficient  to  kill  the  spores. 

In  pasteurization  no  attempt  is  made  to  kill  the  more  resistant 
spores,  but  simply  to  kill  the  vegetative  cells  of  certain  disease 
germs.  For  this  purpose  milk  or  other  substances  to  be  pasteur- 
ized are  heated  to  about  65°  or  75°  C.  By  this  means  fer- 
mentation or  any  other  change  induced  by  active  bacteria  in  the 
pasteurized  material  is  temporarily  checked,  and  the  pasteurized 
milk  or  food  will  thus  remain  unchanged  for  a  longer  period  of 
time.  The  ordinary  disease  germs  which  occur  in  milk,  as  well 
as  the  bacteria  which  sour  milk,  are  killed  by  pasteurization,  and 
the  deleterious  effects  which  result  from  the  high  .temperatures 
necessary  for  sterilization  are  also  avoided. 

Decay.  The  process  commonly  known  as  decay  is  very  largely 
due  to  fermentation  or  to  digestion  by  bacteria  and  other 
saprophytic  fungi  that  live  and  feed  on  the  organic  material 
which  exhibits  the  appearance  and  phenomena  of  decay.  The 
bacteria  which  cause  decay  are  saprophytes,  and  like  the  yeasts 
they  secrete  digestive  and  energy-forming  ferments  which  at- 
tack the  cell  walls  and  other  lifeless  parts  of  animal  and  plant 
remains  upon  which  they  live.  Parts  of  these  organic  remains 
are  thus  fermented  and  digested,  with  the  result  that  the  entire 
form  and  structure  of  the  part  attacked  is  often  destroyed  and 
crumbles  away.  Accompanying  this  fermentation  and  digestion 
of  organic  matter,  gases,  such  as  ammonia  and  sulphureted  hy- 
drogen, are  produced.  In  the  case  of  the  flesh  of  animals  or  of 
material  containing  a  large  percentage  of  nitrogenous  com- 
pounds the  gases  liberated  give  rise  to  odors  which  we  associate 
with  putrefaction.  Nitrates  and  sulphates  are  also  formed  which 
are  carried  back  into  the  soil,  and  these  compounds  ultimately 
serve  as  raw  food  elements  absorbed  by  the  roots  of  green  plants. 
The  decay  of  leaves  and  tree  trunks  in  a  forest,  already  mentioned, 
is  a  good  instance  of  decay  produced  by  saprophytic  bacteria 
and  other  fungi,  and  similar  activities  of  such  bacteria  in  liquids 
are  familiar  phenomena.  The  souring  of  milk  is  caused  by  a 


256  GENERAL  BOTANY 

ferment  secreted  by  a  saprophytic  bacterium  which  causes  the 
sugar  of  milk  to  be  transformed  into  lactic  acid. 

Milk  sugar  4-  lactic  ferment  =  lactic  acid  +  lactic  ferment. 

Similarly,  cider  is  changed  into  vinegar  by  certain  ferments 
secreted  by  bacteria.  This  process  is  brought  about  by  the  oxi- 
dation of  alcohol  in  the  cider  to  form  the  acetic  acid  of  vinegar. 

Alcohol  -f  oxygen  +  acetic  ferment 

=  water  +  acetic  acid  -f-  acetic  ferment. 

Many  other  familiar  instances  might  be  given  in  which  sapro- 
phytic bacteria  cause  profound  changes  in  organic  bodies  by  the 
secretion  of  ferments  which  enable  the  bacteria  themselves  to 
digest  food  or  produce  energy  for  their  own  life  processes. 

Disease.  Certain  parasitic  bacteria,  commonly  designated  as 
disease  germs,  cause  diseases  in  animals  and  plants  by  the  secre- 
tion of  poisonous  substances  called  toxins.  The  toxins  are 
products  manufactured,  like  ferments,  by  the  protoplasm  of  the 
bacterial  cells.  These  toxins  then  diffuse  out  into  the  blood  and 
into  the  tissue  cells  of  the  organism  attacked  by  the  bacteria. 
When  thus  liberated  in  the  system  of  another  organism,  the 
toxins  cause  the  various  symptoms  characteristic  of  particular 
bacterial  diseases.  The  toxins  resemble  the  ferments,  or  enzymes, 
in  being  cell  secretions ;  but  they  do  not  cause  fermentations  or 
digestion,  and  they  have  no  known  function  for  the  bacterial 
cells  secreting  them.  Similar  poisonous  toxins  are  secreted  by 
snakes,  by  the  castor  bean,  and  by  poisonous  mushrooms.  The 
toxins  are  not,  therefore,  confined  to  bacteria,  although  the  bac- 
terial toxins  are  the  most  important  on  account  of  the  diseases 
which  they  cause  in  man,  other  animals,  and  plants.  A  large 
number  of  plant  diseases  are  now  known  to  be  due  to  bacteria. 

Antitoxins  are  secretions  of  the  cells  of  the  organism  attacked 
by  toxic  bacteria,  which  tend  to  neutralize  the  toxins  secreted 
by  the  bacterial  cells.  Diphtheria,  for  instance,  is  produced  by 
bacteria  growing  on  the  lining  membrane  of  the  throat.  The 
diphtheric  bacteria  do  not  enter  the  body  of  the  diseased  organ- 
isms, but  secrete  the  diphtheric  toxin,  which  is  circulated  by 


THE  FUNGI 


257 


the  blood  and  causes  the  symptoms  of  the  disease  in  man  and 
animals.  To  counteract  the  effect  of  the  toxin  the  body  cells 
of  the  diseased  person  secrete  antitoxins,  which,  if  secreted  in 
sufficient  amount,  can  neutralize  the  effects  of  the  diphtheric 
toxin  and  thus  arrest  or  stop  the  progress  of  the  disease.  Other 


FIG.  138.   Bacterial  colonies  growing  on  a  gelatin  culture 

A  sterilized  plate  of  gelatin  exposed  for  five  minutes  in  the  hallway  of  a  school- 
room, and  then  closed  for  five  days,  developed  the  above  colonies.  This  illustrates 
the  dissemination  of  bacteria  in  the  air  and  the  danger  of  increasing  the  normal 
number  by  stirring  up  the  dust  on  the  floor  of  a  room.  From  Bergen  and  Caldwell's 
"  Introduction  to  Botany  " 

antitoxins  produced  in  animals  may,  as  in  the  case  of  diphtheria, 
be  injected  into  the  blood  of  the  diseased  person  and  thus  aid 
in  checking  the  progress  of  the  disease.  Antitoxin  treatment  is 
especially  helpful  in  diphtheria  and  tetanus,  or  lockjaw. 

The  ptomaines  are  complex  chemical  bodies  which  develop  in 
various  organic  substances,  including  canned  foods,  ice  cream, 
etc.  They  are  sometimes  poisonous  and  sometimes  harmless,  and 
must  not  be  confused  with  the  toxins,  which  are  definite  poisonous 


258  GENERAL  BOTANY 

secretions  giving  rise  to  disease.  The  ptomaines  are  regarded  by 
some  as  bacterial  secretions  and  by  others  as  by-products  of  the 
decomposition  caused  in  organic  material  by  various  saprophytic 
bacteria.  Ptomaine  poisoning  is  more  likely  to  occur  in  warm 
weather,  since  conditions  are  then  more  favorable  to  general 
bacterial  activity. 

SUMMAEY 

From  the  above  discussion  it  has  been  found  not  only  that  bac- 
teria have  the  same  general  cell  structure  as  other  organisms  but 
that  their  physiological  activities  and  products  closely  resemble 
those  of  the  yeasts  and  other  saprophytic  and  parasitic  fungi  to 
which  they  are  most  closely  related.  The  decay  and  putrefaction 
caused  by  bacteria  is  indirectly  a  result  of  certain  digestive  and  fer- 
mentative processes  which  belong  in  the  same  category  as  similar 
nutritive  processes  occurring  in  yeasts  and  other  fungi.  Even  the 
disease-producing  power  of  bacteria  is  due  to  products  of  their  cell 
protoplasts,  which  are  not  peculiar  to  the  bacterial  cells  alone.  The 
striking  effects  of  bacteria  on  other  organisms  and  upon  organic 
material  are  due,  therefore,  to  their  great  numbers  and  to  the  im- 
mense scale  upon  which  they  act,  rather  than  to  any  marked  pecu- 
liarity either  in  their  organization  or  in  their  physiological  processes. 

MOLDS 

Everyone  is  familiar  with  the  appearance  of  molds  on  bread, 
jelly,  and  other  household  products.  The  white,  fluffy  character 
of  the  molds  is  due  to  the  innumerable  colorless  filaments 
which  comprise  the  plant  body.  These  filaments  are  individually 
called  hyphce,  while  the  entire  white  mass  of  filaments  consti- 
tuting a  mold  colony  is  called  a  mycelium.  The  hyphse  of  the 
molds,  and  other  fungi  as  well,  resemble  very  closely  the  fila- 
ments of  green  algse  like  Vaucheria  or  Spirogyra,  except  that 
they  lack  chloroplastids  and  green  chlorophyll.  The  mycelium 
which  is  formed  by  them  usually  occurs  both  on  the  surface  and 
within  the  substances  upon  which  they  grow.  The  term  aerial 
hyphce  or  aerial  mycelium  is  applied  to  the  visible  surface  filaments 
of  a  mold,  while  those  which  grow  within  a  substance  are  called 
submerged  hyphce  or  submerged  mycelium  (Figs.  139  and  140). 


THE  FUNGI 


259 


This  differentiation  of  the  plant  body  of  a  mold  into  an  aerial 
and  a  submerged  portion  has  an  important  physiological  bearing 
on  the  life  of  the  plant.  The  aerial  hyphse  give  rise  to  the  spores, 
which  are  thus  in  a  position  to  be  readily  disseminated  in  the 
air,  while  the  submerged  hyphse  ramify  through  the  nutrient 


FIG.  139.   The  habit  of  the  black  mold  (Rhizopus)  growing  on  bread 

The  black  heads  are  the  sporangia  growing  from  the  aerial  mycelium 
of  the  mold.   After  Conn 

medium  on  which  the  mold  is  growing,  and  both  digest  and 
absorb  food  for  the  entire  fungus  plant.  The  submerged  hyphse 
thus  serve  the  double  function  of  digesting  and  absorbing  foods. 
We  see,  therefore,  that  even  in  these  very  simple  plants  there 
is  a  division  of  labor  which  amply  meets  their  demands  for  food 
and  the  need  for  wide  distribution.  These  facts  will  become 
clearer  by  the  study  of  two  or  three  common  types  of  molds 
selected  from  two  different  groups  of  fungi.  The  black  molds, 
represented  by  Rhizopus,  belong  to  the  lowest  group  of  algal 


260  GENERAL  BOTANY 

fungi,  while  the  blue-green  molds  belong  to  the  sac  fungi. 
They  are,  however,  conveniently  studied  together  on.  account  of 
the  similarity  in  their  general  structure  and  mode  of  life. 

/The  black  molds  are  readily  distinguished  by  the  naked  eye 
on  account  of  the  dark  spore  cases,  or  sporangia,  which  are  borne 
by  the  older  portions  of  the  aerial  mycelium.  If  examined 
closely,  these  sporangia  look  like  black  spheres  supported  upon 
minute  stalks,  or  hyphse,  springing  from  the  white  aerial  mycelium 
of  the  mold.  The  younger  sporangia  are  white,  becoming  gradu- 
ally darker  with  age.  Another  characteristic  of  the  black  molds 
is  the  peculiar  structure  of  the  hyphal  filaments,  which,  though 
long  and  frequently  branched,  are  yet  composed  of  single  cells, 
like  Vaucheria,  without  partitions  or  septa  but  with  many 
nuclei.  They  have  been  likened  to  a  greatly  elongated  yeast 
cell  which  with  its  growth  has  failed  to  form  new  cell  walls 
to  inclose  the  repeatedly  dividing  nuclei  of  the  growing  cell. 
A  convenient  species  for  study  is  the  common  black  mold  of 
bread,  known  as  JKhizopus  niyru-am. 

RmZOPUS  NIGRICANS   (BLACK    MOLD) 

Habit.  Rliizopus  has  the  same  branched  multinucleate  and 
unicellular  hyphse  as  the  rest  of  the  black  molds.  It  is  peculiar 
in  that  it  spreads  over  the  surface  of  bread  or  other  nutrient 
media  by  means  of  special  hyphse  termed  runners  or  stolons. 
The  mold  spreads  by  these  stolons  in  much  the  same  manner  as 
strawberry  plants  spread  by  runners  growing  out  from  the 
mother  plants.  The  stolons  are  hyphse  which  grow  out  radially 
from  centers  where  the  mycelium  is  already  established.  At  cer- 
tain intervals  these  hyphal  stolons  send  out  rootlike  submerged 
hyphal  branches,  which  penetrate  the  nutrient  medium  on  which 
it  grows  and  anchor  the  aerial  mycelium.  The  sporangia  and 
spores  are  borne  on  erect  aerial  hyphse  which  spring  from  the 
points  on  the  stolons  where  the  rootlike  hyphse  grow  out.  The 
sporangia  and  spores  are  thus  borne  at  the  most  favorable  points 
for  receiving  food  from  the  submerged  hyphse,  as  these  digest 
and  absorb  the  starch  and  other  nutrient  substances  of  the  bread. 


THE  FUNGI 


261 


Nutrition.  Rliizopus  is  able  to  grow  not  only  upon  bread  but 
upon  a  great  variety  of  organic  material  if  kept  in  moist  places. 
From  some  substances  it  is  undoubtedly  able  to  absorb  foods 
which  are  already  in  solution,  while  in  other  cases,  as  on  breads, 
much  of  the  food  material  is  insoluble  and  must  first  be  digested 
by  the  submerged  hyphse  before  it  can  be  absorbed  and  used 
by  the  cells  of  the  mold  mycelium  or  by  the  sporangia  during 
spore  formation  (Fig.  140).  In  order  to  digest  the  food  the  sub- 
merged hyphse  secrete  digestive  ferments  similar  to  those  already 
discussed  under  yeasts.  These  digestive  ferments  diffuse  out  of 
the  hyphse  and  con- 

Sporcs 
*  !•"•  • 

Columella' 


vert  the  starches,  fats, 
and  proteins  of  the 
bread  on  which  the 
mold  is  growing  into 
soluble  and  diffusi- 
ble foods.  In  other 
words,  the  mold  di- 
gests its  food  much 
as  higher  plants  do, 
except  that  the  di- 


,8pores 
.Columella 


Spor 
Mycelium, 


FIG.  140.    A  drawing  illustrating  the  growth  of 


Ehizopus  on  and  within  a  piece  of  bread 
gestioil    in    the   mold      The  absorbing  and  feeding  mycelium,  composed  of 
takes    place    entirely      branched  hyphse,  is  shown  within  the  bread;   also  a 
.    .  ,         f   , ,        ,      -,  stolon  and  two  groups  of  sporangia 

outside   of  the  body 

cells  of  the  plant  and  within  the  nutrient  medium  immediately 
surrounding  its  hyphse.  Some  of  the  black  molds,  in  addition 
to  the  digestive  enzymes,  are  able  to  secrete  enzymes  which 
bring  about  fermentations  in  sugar  solutions  exactly  like  those 
of  the  yeast. 

Asexual  reproduction.  The  hyphse  which  bear  the  sporangia 
and  spores  in  RJiizopus  arise  at  the  point  of  origin  of  the  root- 
like  outgrowths  which  spring  from  the  stolons,  where  they  first 
grow  out  as  short,  erect  branches  of  the  mycelium  (Fig.  141). 
These  aerial  hyphse  soon  begin  to  swell  at  the  ends,  producing 
a  light-colored  spherical  enlargement  which  is  the  beginning 
of  the  future  sporangium.  The  entire  structure,  including  the 
erect  hypha  and  its  swollen  end,  is  now  called  a  sporangiophore, 


262 


GENERAL  BOTANY 


or  sporangium  bearer.  A  cell  wall  next  appears,  separating  the 
young  sporangium  from  the  hypha  which  bears  it,  thus  forming 
a  greatly  swollen  terminal  and  spherical  sporangial  cell  which 
is  filled  with  dense  cytoplasm  and  many  nuclei.  This  sporangial 
cell  now  expands  with  great  rapidity,  and  with  its  expansion  the 
wall  separating  it  from  its  hyphal  stalk  grows  in  surface  area  and 
assumes  a  convex  form,  protruding  into  the  growing  sporangium 
until  it  comes  to  occupy  fully  one  half  or  two  thirds  of  the  spo- 
rangial cavity  (5),  Avhen  it  is  called  the  columella.  The  mold,  if 

Youny  sporangia     Mature 

I    M^l         sporangium 


FIG.  141.    Development  of  the  sporangia  and  spores  in  Rhizopus 

a,  young  sporangia  occurring  as  terminal  enlargements  of  aerial  hyplue;    6,  older 
sporangium  with  columella  and  spores  differentiated ;   c,  spores  being  shed  by  rup- 
ture of  sporangium  wall ;  d,  early  stage  of  spore  germination ;  e,  later  stage,  with 
branched  hyphal  filament 

observed  at  this  stage  with  a  hand  lens,  appears  to  be  dotted 
with  round  white  or  gray  balls,  which  are  the  young  sporangia. 
Meanwhile  internal  cell  processes  which  are  to  result  in  the  for- 
mation of  the  spores  have  been  going  on  in  the  cytoplasm  of  the 
sporangium.  As  the  spores  mature  they  secrete  a  black  pigment, 
so  that  the  sporangia  become  darker  with  age.  Finally  the  spo- 
rangium cavity  is  filled  with  innumerable  dark  spores,  and  the 
plant  now  presents  the  familiar  appearance  of  the  black  molds, 
owing  to  the  large  number  of  black  sporangial  heads  which  cover 
its  surface.  When  the  sporangia  are  ripe,  the  sporangium  wall 
ruptures  by  drying,  and  the  light  spores  are  widely  scattered  by 
wind  in  the  open  and  by  air  currents  in  dwellings. 


THE  FUNGI 


263 


Suspensors 

r 


TJie  germination  of  the  spores  takes  place  as  follows :  When 
they  come  to  rest  on  a  proper  nourishing  medium,  the  spores 
absorb  water,  the  outer  coat  ruptures,  and  the  delicate  inner  coat 
expands  into  a  tubular  hypha,  into  which  the  protoplasm  of  the 
spore  flows  (Fig.  141,  d  and  e).  As  the  hypha  elongates  it  ab- 
sorbs new  nutriment,  by  means  of  which  it  grows  and  branches 
to  form  a  new  plant  body,  or  mycelium.  Usually  several  spores 
germinate  in  close  proximity,  and  the  hyphse  produced  by  the 
different  spores  commingle  to  make  one  fluffy, 
white  mycelium  characteristic  of  the  black  molds. 

Sexual  reproduction.  Under  special  conditions 
two  hyphae,  belong- 
ing to  separate  my- 
celia  of  different 
kinds,  or  strains, 
may  conjugate  and 
produce  a  zygote. 
Each  hypha  sends 
out  a  fertile  hyphal 
branch,  or  bud, 
called  a  suspensor. 
The  two  branches 
grow  toward  each 
other  in  much  the 
same  way  as  the 
conjugating  tubes 

do  in  Spirogyra,  and  each  suspensor  cell  then  cuts  off  at  its 
apex  a  cell  which  corresponds  to  a  gametangium  (Fig.  142). 
The  two  end  cells,  or  gametangia,  of  the  approaching  suspensors 
meet,  and  the  protoplasts  (gametes)  unite  after  the  solution  and 
disappearance  of  the  cell  walls  of  the  two  gametangia  at  the  point 
of  contact.  The  zygote  cell  thus  formed  enlarges  and  forms  the 
usual  heavy  protective  walls  of  a  zygote.  After  a  period  of  rest 
this  zygote  may  germinate  and  produce  a  new  mycelium  (<?),  from 
which  sporangia  and  asexual  spores  will  be  formed  as  described 
above.  Although  the  sexual  reproduction  of  Rhizopus  and  Spi- 
rogyra  are  similar,  the  two  plants  have  no  near  relationship. 


A'  -  - 
Gametes 


Suspensors 
b 


FIG.  142.    Sexual  reproduction  in  Rhizopus 

a,  formation  of  the  suspensors  and  gametangia ;  b,  forma- 
tion of  the  zygote ;  c,  germination  of  the  zygote  to  form 
a  new  sporangium 


264 


GENERAL  BOTANY 


PENICILLIUM  (BLUE  MOLD) 

Habit.  The  common  blue-green  molds  are  found  on  decaying 
fruits,  vegetables,  and  foods  of  various  kinds.  They  differ  from 
the  black  molds  in  that  their  hyphse  are  subdivided  into  many 
cells,  each  cell  of  which  is  provided  with  a  single  nucleus. 
They  also  produce  asexual  spores  by  the  transverse  division  of 
special  spore-bearing  hyphse  and  not  by  cell  division  within  a 

sporangium,  as  in  Rhizopus. 
When  fruits  or  other 
nutrient  media  suitable  for 
the  growth  of  Penicillium 
are  left  in  a  moist  place, 
white  patches  of  mold  are 
almost  certain  to  appear  in 
two  or  three  days  on  the 
surface  of  the  exposed  sub- 
stance (Fig.  143).  Careful 
examination  of  these  areas 
will  reveal  more  or  less 
radiate  patches  of  hyphse 
constituting  the  young 
mold  colonies  of  Penicil- 
lium. This  radiate  appear- 
ance will  be  found  to  be 
due  to  the  fact  that  the 
new  hyphse  which  make  up  the  white  mycelium  of  the  mold 
are  growing  out  centrifugally  from  the  point  where  the  mold 
spores  started  to  germinate.  This  centrifugal  growth  continues 
until  neighboring  patches  unite  into  one  extensive  mycelium, 
thus  obscuring  the  method  of  origin. 

The  patches  of  mycelium  described  above  start  from  spores 
which  have  been  deposited  from  the  air  on  any  given  nutrient 
medium.  These  spores  germinate,  like  those  of  Mucor  already 
described,  by  the  rupture  of  the  outer  coat  of  the  spore  and  the 
protrusion  of  the  inner  coat  to  form  a  hyphal  filament.  If 
spores  of  Penicillium  are  sown  on  prune  juice  or  on  some  other 


FIG.  143.   Decay  of  apples  caused  by 
mold  growth 

The  mycelium  of  the  mold  penetrates  the  apple, 

as  in  the  case  of  Rhizopus  on  bread  (Fig.  140), 

and  breaks  down  its  substance,  thus  causing 

decay.   After  Conn 


THE  FUNGI 


265 


O 


favorable  medium  and  are  then  placed  in  a  suitable  temperature 
(15°-30°  C.)  for  twenty-four  hours,  all  stages  in  the  germina- 
tion process,  can  be  observed  with  a  sufficient  magnification. 
The  elongation  of  the  inner  coat  to  form  a  hypha  is  accom- 
panied by  nuclear  and  cell  division,  so  that  the  resulting  hyphse 
and  mycelium  are  multicellular.  The  individual  hyphse  have  a 
delicate  cell  wall  with  highly  granular  vacuolate  cytoplasm,  and 
repeatedly  give  off  branches,  ultimately 
forming  the  patches  of  mold  already  de- 
scribed. In  nature  and  in  the  artificial 
sowings  of  spores  more  than  one  spore 
usually  starts  a  mycelium  at  a  given  point. 
The  centrifugal  method  of  growth  which 
follows  is  advantageous  in  bringing  the  my- 
celium in  contact  with  more  of  the  nutrient 
material  and  in  forming  a  larger  surface 
for  the  formation  of  spores.  As  growth 
proceeds  submerged  hyphse  grow  down 
into  the  fruit  or  other  nutritive  medium 
and  perform  the  functions  of  digesting 
and  absorbing  food  for  the  entire  plant. 

Asexual  reproduction.  If  patches  of 
Penicillium  are  examined,  it  will  be  seen 
that  they  gradually  change  their  color  from 
pure  white  to  sage  green.  This  change  in 
color  usually  begins  in  the  older  central  portion  of  the  cir- 
cular patches  of  mycelium  and  spreads  from  the  center  toward 
the  circumference  of  any  given  mold  colony.  It  is  due  to  the 
formation  of  innumerable  spores,  which,  taken  in  the  mass, 
are  green  in  color.  The  spores  are  formed  upon  erect  aerial 
hyphse,  which  branch  repeatedly  at  the  end,  forming  a  brush- 
like  or  treelike  growth  of  short  hyphal  branches  (Fig.  144). 
Each  branch  then  begins  to  constrict  below  the  apex  just  as 
though  a  thread  had  been  tied  around  the  hypha  at  this  point 
and  then  gradually  tightened.  This  process  is  repeated  until 
each  hyphal  branch  of  the  main  spore-bearing  hypha  is  con- 
verted into  a  row  of  spores.  The  older  spores  at  the  ends  of 


FIG.  144.    The  method 

of  producing  spores  in 

Penicillium 

Consult  the  text  for  a 
discussion  of  the  spore- 
forming  process.  At  the 
right  are  germinating 
spores.  After  Thorn 


266 


GENERAL  BOTANY 


the  branches  break  off  and  are  disseminated  by  air  currents,  thus 
scattering  the  mold  spores  widely.  The  large  number  of  spores 
produced,  and  the  great  resistance  of  the  spores  to  extremes  of 
temperature  and  moisture,  account  in  part  for  the  wide  distri- 
bution of  Penicillium  in  nature. 


ASPEEGILLUS 

Aspergillus  is  another  blue-green  mold  which  often  occurs 
with  Penicillium  in  cultures,  and  differs  from  it  largely  in  the 
nature  of  the  spore-bearing  hyphse  (Fig.  145).  These  hyphse 

in  Aspergillus  swell 
at    the    end   into    a 
large,   spherical  cell 
which   looks   like   a 
young      sporangium 
of    Hhizopus.      The 
swollen  end  cell  then 
buds  out  into  innu- 
merable short,  radiat- 
ing   hyphse,     which 
together  form  a  com- 
pact head.    Each  ra- 
diating  hypha  then 
constricts    to     form 
a    chain    of    spores, 
exactly   as   in  Peni- 
cillium.   In  other  re- 
spects Aspergillus  is  scarcely  distinguishable  from  Penicillium  and 
has  essentially  the  same  general  appearance  and  biologic  history. 
Both  Penicillium  and  Aspergillus  produce  gametes  which  fer- 
tilize  and   produce   spore   fruits.      The   sexual    process    in   the 
molds  and  in  many  other  fungi  seems,  however,  to  have  become 
quite   subordinate   to   the   asexual   spore    process,   upon   which 
these  fungi  depend  almost  entirely  for  their  perpetuation  and 
wide  distribution. 


Spores 


FIG.  145,    A  colony  of  Aspergillus,  showing 
mycelium  and  spore  clusters 

The  lower  figures  show  in  detail  the  method  of  spore 
formation.   After  Conn 


THE  FUNGI 


267 


MUSHROOMS  AND  THEIR  ALLIES 
MUSHROOMS 

The  mushrooms  belong  to  the  higher  fungi  and  include 
among  their  allies  the  puffballs,  the  bracket  fungi,  and  the 
rusts  and  smuts  of  cereal  grains. 

Habit.  The  plant  body  of  a  mushroom  is  divided,  like  that 
of  the  molds  already  studied,  into  an  aerial  portion  (the  mush- 
room proper),  which  bears  spores  (Fig.  146),  and  a  submerged 


FIG.  14&.   Habit  of  the  "  shaggy  mane  "  mushroom,  Coprinus  comatus 
After  Buller 

mycelium,  which  digests  and  absorbs  food  from  the  organic 
matter  in  the  soil.  The  mushrooms  are  therefore  saprophytes 
and  grow  best  around  old  stumps  or  in  soil  which  is  rich  in 
decaying  vegetable  remains.  The  mycelium,  often  called  spawn, 
traverses  this  soil  in  the  form  of  long  white  threads  made  up  of 
strands  of  mycelial  fibers  closely  interwoven.  This  spawn  may 
live  for  several  years,  and  produces  each  season  a  new  crop 
of  mushrooms.  The  mushroom  plant  above  soil  is  therefore  a 


268  GENERAL  BOTANY 

kind  of  fruit  body,  which  expands,  produces  spores,  and  dies 
down  again  in  a  few  days,  while  the  real  lasting  and  perennial 
portion  of  the  plant  is  the  feeding  submerged  mycelium  within 
the  soil. 

The  spore-bearing  mushroom  fruit  is  definitely  adapted  to 
the  production  and  dispersal  of  large  numbers  of  asexual 
'spores.  Its  parts  are  the  stipe,  or  stalk,  the  pileus,  or  umbrella, 
and  the  lamellce,  or  gills.  The  stipe  bears  the  pileus  at  its  apex 
and  serves  to  lift  it  into  the  air  when  the  spores  are  ripened  and 
ready  to  be  shed.  The  pileus  bears  the  lamellae,  or  gills,  on 

Pileus 

55*^  .  ^S^^^^^s^ 

Pileus 

I 

Annulus 
Stipe 


Mycelium 


FIG.  147.    Diagrammatic  figures  of  young  and  mature  mushrooms 
growing  in  the  soil 

Note  the  soil  mycelium  similar  to  that  of  Rhizopus  in  bread  (Fig.  140).    Adapted 
from  Buller's  "Researches  on  Fungi " 

its  under  surface  as  radiating  plates  which  extend  from  the 
stipe  to  the  margin  of  the  pileus  and  greatly  increase  the  spore- 
bearing  surface  of  the  mushroom  fruit,  since  the  spores  are 
borne  over  the  entire  surface  of  each  lamella.  Buller  estimated 
that  a  single  lamella  of  Coprinus  comatus  produced  about 
24,480,000  spores  and  that  all  of  the  lamellae  of  this  mushroom 
bore  approximately  5,240,000,000  spores. 

When  the  mushroom  is  young  (Fig.  147),  the  pileus  is  bent 
down  like  a  folded  umbrella,  thus  protecting  the  young  lamellae 
and  spores  during  the  early  stages  of  their  development.  Some 
mushrooms  have  a  further  protection  for  the  lamellae  and  spores 
in  the  form  of  a  delicate  veil  composed  of  hyphae  which  stretch 
from  the  edge  of  the  pileus  to  the  young  stipe,  so  that  the  lamellae 
are  inclosed  in  a  chamber  away  from  the  outside  air.  As  the 


THE  FUNGI 


269 


mushroom  matures,  the  stipe  elongates  and  the  pileus  expands, 
so  that  the  lamellae  are  lifted  into  the  air  and  properly  disposed 
for  the  dissemination  of  the  spores.  At  the  same  time  the  veil 
is  ruptured,  and  a  remnant  of  it  clings  to  the  stipe  as  a  ring  of 
tissue,  called  the  annulus,  marking  the  former  junction  of  the 


Basidiiim 


Pileus — 
Lamella — , 


Spores 


^..'Paraphysis 


FIG.  148.    Spore  formation  in  a  mushroom,  Coprinus 

a,  vertical  section  of  the  mushroom  fruit;  b,  vertical  section  of  three  lamellae,  or 
gills  (the  arrows  indicate  the  path  of  the  spores  when  shot  off  from  the  sterigmata) ; 
c,  d,  portions  of  the  hymenium  showing  spores  on  the  basidia ;  c,  surface  view  of  the 
hymenium  ;  d,  sectional  view.  Further  discussion  in  the  text,  b,  c,  d,  redrawn  from 
Buller's  "  Researches  on  Fungi  " 

veil  with  the  stipe.  The  output  and  dissemination  of  spores  is 
also  greatly  facilitated  by  the  structure  of  the  lamellae  and  by 
the  method  of  producing  and  freeing  the  spores  by  the  specialized 
spore-producing  cells,  or  basidia. 

Asexual  reproduction.  The  lamellae  are  formed  by  hyphae 
which  grow  downward  from  the  pileus  and  unite  to  form  the 
various  layers  of  the  gills. 

The  central  portion  of  a  lamella  is  made  up  of  vertically 
arranged  hyphae  which  unite  by  cross  branches  to  form  the 


270 


GENERAL  BOTANY 


trama.  The  hyphge  of  this  central  trama  turn  outward  toward 
the  surface  of  the  lamella  and  end  in  large,  club-shaped  cells 
which  form  the  hymenium,  or  spore-bearing  layer  (Fig.  148,  d). 
This  hymenium  covers  the  entire  surface  of  the  lamella  like  an 
epidermis,  and  is  composed  of  two  kinds  of  club-shaped  cells, 

namely,  the  fertile  cells ; 
called  basidia,  which  form 
the  spores,  and  the  sterile 
cells,  or  paraphyses,  which 
separate  the  basidia  at  cer- 
tain intervals.  In  a  surface 
view  of  a  lamella  the  ends 
of  the  sterile  cells  and  of 
the  spore-bearing  basidia 
are  seen.  If  such  a  view 
is  taken  of  a  gill  under 
the  low  power  of  a  com- 
pound microscope,  an  ap- 
pearance like  that  in 
Fig.  148,  <?,  is  shown. 
The  sterile  cells  now  ap- 
pear as  large,  circular, 
light-colored  cells,  while 
the  spores  borne  by  the 
basidia  are  seen  in  groups 
of  four  small,  dark  bodies. 
This  view  also  gives  one 
a  good  idea  of  the  con- 
tinuous epidermislike  character  of  the  hymenium.  When  the 
hymenium  begins  to  bear  spores,  each  basidium  buds  out  at 
its  large,  free  end  into  four  slender  protrusions  of  the  cell  wall, 
called  sterigmata,  and  each  sterigma  then  swells  up  at  the  end 
into  a  spherical  spore  cell,  into  which  cytoplasm  and  a  nucleus 
flow  from  the  main  body  of  the  basidium.  The  spores  change  in 
color  as  they  mature,  and  cause  a  change  in  the  color  of  the  gill 
from  white  to  black,  broAvn,  or  pink  in  many  species  of  mush- 
rooms. When  they  are  ripe  they  are  shot  off  from  the  sterigmata 


FIG.  149.  Two  species  of  puffballs,  the  upper 
one  shedding  spores 


THE  FUNGI 


271 


and  fall  vertically  in  the  space  between  the  two  adjacent  gills. 
Buller  estimates  the  spore  discharge  in  Coprinus  comatus,  the 
common  shaggy  Coprinus,  to  aggregate  one  hundred  million 
spores  per  hour  during  the  period  of  active  spore  discharge,  so 
that  if  any  considerable  proportion  of  these  spores  germinated 
and  produced  new  plants,  the  world  would  very  soon  be  overrun 
with  mushrooms.  Most  of  these  reproductive  bodies  of  the  mush- 
room fail  to  grow,  however,  on  account  of  an  unfavorable  environ- 
ment, so  that  there  is  no  perceptible  increase  of  the  species. 


PUFFBALLS 

The  puffballs  resemble  the  mushrooms  in  being  saprophytes. 
The  plant  is  also  divided,  like  the  mushrooms  and  molds,  into 
an  underground  feeding  mycelium,  or  spawn,  and  an  aerial  por- 
tion which  bears  spores.  The  spawn  is  identical  in  appearance 

Spore 
chamber 


Mycelium  (spawn) 

FIG.  150.   Drawing  showing  the  external  and  internal  structures  of  a  puffball 

a,   external  view   with  mycelium   in   the   soil;    b,  median  long   section  showing 

gleba  and   peridmm;    c,   ripe  puffball,   in  which   the   gleba  is  transformed   into 

spores  and  capillitium 

with  that  of  the  mushroom,  but  the  aerial  fruit  body  is  the  sub- 
spherical  puffball  seen  in  nature  (Fig.  150).  The  hyphae  of  which 
the  puffball  is  composed  differentiate  very  early  into  one  or  more 
dense  outer  layers,  called  the  peridia  (singular,  peridium),  and  an 
inner  loose  mass  of  hyphse,  called  the  gleba.  This  inner  gleba 


272 


GENERAL  BOTANY 


then  forms  spore  cavities  which  are  lined  with  a  continuous 
hymenial  layer  like  that  which  covers  the  outer  exposed  surface 
of  a  mushroom  lamella.  When  the  spores  are  ripe,  the  hymenium 
and  the  hyphse  of  the  trama,  on  which  the  hymenia  of  the  several 
spore  cavities  are  borne,  break  down,  and  the  spores  become  free 

within  the  peridium,  or 
outer  covering.  The  dry- 
ing of  the  entire  disor- 
ganized portion  of  the 
gleba  and  the  growth  of 
the  spores  result  in  the 
formation  of  a  large  cen- 
tral spore  cavity  filled 
with  ripe  spores.  Some 
of  the  hyphse  of  the  gleba 
may  also  thicken  to  form 
long-branched  threads,  the 
capillitium  threads,  which 
appear  among  the  ripe 
spores  in  some  species. 
The  rupture  of  the  outer 
gleba  by  drying,  by  decay, 
or  by  mechanical  means 
results  in  the  wide  dis- 
semination of  the  spores 
by  the  wind.  The  puff- 
ball  is  not,  therefore,  as 
completely  adapted  to 

spore  dissemination  as  the  mushroom,  in  which  the  elongated 
stipe  and  the  exposed  lamellae  facilitate  spore  scattering. 


FIG.  151.    Parasitic  fungus,  Pleurotus  ulma- 
rius,  growing  in  a  wound  on  a  maple  tree 

After  E.  M.  Freeman 


BRACKET  FUNGI 

The  bracket  fungi  (Fig.  151)  are  also  closely  related  to  the 
mushrooms  and  puffballs  on  account  of  their  method  of  forming 
spores  by  means  of  basidia  arranged  in  the  form  of  hymenial 
layers.  These  hymenial  layers,  in  some  species,  cover  either 


THE  FUNGI 


273 


smooth  or  greatly  roughened  surfaces,  but  in  the  more  common 
species  they  line  pores  on  the  under  side  of  the  shelflike  fruit 
body.  Since  these  pores  open 
out  on  the  under  surface  of  the 
fungus,  the  spores  are  readily 
disseminated,  as  in  the  mush- 
room, by  being  shot  off  from 
the  basidia  into  the  open  space 
of  the  pores,  from  which  they 
fall  downward  into  the  open  air. 
The  bracket  fungi  are  often 
very  hard  and  woody,  but  sec- 
tions cut  from  the  plant  body 
show  that  it  is  composed  en- 
tirely of  thickened  hyphse. 
The  feeding  mycelium  pene- 
trates and  absorbs  food  from 
rotten  logs  and  stumps  in  the 
saprophytic  species,  but  in  the 
parasitic  forms,  which  are  so 
destructive  to  forest  trees,  the 
mycelium  penetrates  into  the 
living  tissues  of  the  tree  and 
destroys  it  (Fig.  152).  This 
penetration  of  the  hyphse  into 
the  hard  wood  of  trees  usually 
begins  at  a  wound  on  the  ex- 
posed surface  of  the  trunk  or 
branches,  and  its  entrance  into 
the  tree  is  effected  by  the  se-  The  fisure  shows  the  mycelium  of  Polv- 

*  porus  squamosus  penetrating  and  break- 

cretion  of  Cellulose  and  wood  ing  down  the  wood-cell  walls  by  digestion, 
ferments,  which  digest  a  pas-  From  Duggar's  "Plant ^  Diseases."  After 

sage  for  the  hyhse.  Once  within 

the  tree,  these  hyphse  branch  within  its  tissues  and  not  only 
break  down  the  wood-cell  walls  but  attack  the  living  cells  of 
the  tree  and  their  stored  food.  The  bracket  fungi  thus  destroy 
living  forest  trees  and  logs,  railroad  ties,  and  lumber. 


FIG.  152.  Destruction  of  wood  by 
funsus 


274  GENEKAL  BOTANY 

RUSTS  AND  SMUTS 

The  rusts  and  smuts  are  the  most  distant  relatives  of  the 
mushrooms,  puffballs,  and  bracket  fungi.  The  relationship  is 
established  largely  through  their  method  of  producing  spores, 
but  the  evidence  is  too  detailed  for  our  present  discussion. 
They  are  of  great  importance  in  agriculture  on  account  of  the 
destruction  which  they  cause  to  the  various  species  of  cereals 
which  form  the  staple  crops. 

RUSTS  (PUCCINIA  G-RAMINIS) 

This  rust  is  heteroecious,  producing  four  different  kinds  of  spores 
at  different  seasons  of  the  year  and  in  using  two  plant  species 
as  hosts  for  the  growth  and  formation  of  its  spores.  Each  kind 
of  spore  also  plays  a  particular  rc>le  in  the  life  history  of  the 
organism  (Fig.  153). 

The  spring  crop  of  spores,  called  ceciospores  or  ceeidiovpore*,  are 
borne  in  open  cups  (F)  on  the  leaves  of  the  common  barberry. 
These  barberry  spores  are  supported  by  a  mycelium  which  pene- 
trates the  barberry  leaf  as  a  parasite  and  absorbs  food  for  itself 
and  the  spore-bearing  hyphse  produced  within  the  barberry  cups. 
These  spring  spores  become  mature  at  about  the  time  when  the 
young  wheat  plants  are  springing  up  in  the  fields,  and  when  they 
are  blown  by  the  wind  and  fall  upon  a  young  wheat  plant  they 
germinate  and  produce  a  dense  mycelium  within  the  tissues  of  its 
growing  leaves  and  stem.  This  inner  parasitic  mycelium  then 
forms  red-rust  spores,  in  groups  called  sori  (-5),  at  certain  points 
along  the  surface  of  the  leaves  and  the  stem.  These  masses  of 
spores  break  through  the  epidermis  and  form  the  long  lines  of 
spores  (J[)  familiarly  known  as  red  rust. 

These  summer  spores,  called  urediniospores  or  uredospores,  are 
single-celled  and  are  borne  on  a  short  stalk.  They  are  quickly 
disseminated,  and  since  they  germinate  at  once  under  favorable 
conditions,  they  serve  to  spread  the  rust  very  widely  in  the  fields 
of  grain  in  early  summer.  In  warm  regions  the  red-rust  spores 
often  survive  the  winter  and  start  the  rust  in  the  spring. 


FIG.  153.    Wheat  rust  (Puccinia  graminis) 

A,  portion  of  stem  of  wheat  with  rust  spores  in  groups,  or  sori;  B,  sectional  view  of 
a  sorus,  with  young  uredospores  (?/?/)  and  mature  uredospores  (u),  stalks  (st),&nd  my- 
celium (m)  within  the  stem  tissues;  C,  red-rust  spore  (uredospore)  with  stalk  (st) 
sending  out  infection  hyphae  (m) ;  D,  two-celled  black-rust  spore  (teleutospore,  t) 
with  greatly  thickened  cell  wall  and  stalk  (st) ;  E,  germination  of  the  upper  spore 
cell  of  the  teleutospore  (t)  to  form  the  promycelium  (p),  hearing  disseminating 
spores  (sporidia,  s)  •  F,  section  of  barberry  leaf  showing  the  aecidium  cup  with 
spores  (s),  the  wall  of  the  cup,  or  peridium  (p),  and  the  upper  and  lower  epidermis 
of  the  leaf  (e±  and  e2).  Adapted  from  Duggar's  "  Fungous  Diseases  of  Plants  " 


276  GENERAL  BOTANY 

The  black-rust  spores,  called  teliospores  or  teleutospores,  are 
formed  in  the  same  sori  and  from  the  same  mycelium  as  the 
red  rust,  but  are  produced  later  in  the  season  than  the  latter. 
The  black-rust  spores  are  two-celled,  thick-walled  wintering 
spores  (Z>)  which  live  through  the  winter  on  the  straw  or  stubble 
and  germinate  the  next  spring  after  their  production.  Their 
germination  (JS)  results  in  small  spores,  usually  called  sporidia 
(s),  which  are  blown  to  the  young  leaves  of  the  barberry  and 
start  a  mycelium  for  the  production  of  a  fresh  crop  of  aecidium 
cups  and  spores  on  the  barberry  leaves. 

This  completes  the  remarkable  cycle  of  these  peculiar  plants, 
which  have  become  so  highly  adapted  in  their  spore  forms  to 
the  seasonal  life  of  the  organism.  This  seasonal  life,  as  the 
above  account  indicates,  comprises  two  distinct  feeding  mycelial 
plants,  which  grow  on  two  different  species  of  flowering  plants. 
Four  kinds  of  spores  are  also  produced,  which  serve  for  dissemi- 
nating the  plants  in  spring  and  summer  and  for  wintering. 

Other  forms  of  rusts  occur  on  various  cereal  grains  and  on 
the  grasses,  to  which  the  cereals  are  closely  related.  Not  all  of 
these  rusts,  however,  have  so  complicated  a  life  history  as  that 
of  Puecinia  graminis,  although  most  of  them  have  at  least  two 
forms  of  spores  and  many  species  avail  themselves  of  two  hosts 
in  the  production  of  these  spores.  The  rusts  are  among  the 
most  destructive  of  the  fungi  and  cause  the  loss  of  many 
millions  of  dollars  each  year  by  the  damage  to  cereals  of 
various  kinds. 

SMUTS 

The  most  familiar  smut  is  that  of  Indian  corn  (  Ustilago  zeae) 
(Fig.  154),  which  causes  the  distortion  of  the  kernels  of  corn  on 
the  cob  and  produces  the  large  black  masses  of  smut  spores  seen 
in  fields  of  corn  in  the  autumn.  If  the  smutted  ears  of  corn  are 
examined  early  in  the  season,  it  will  be  found  that  the  fungus 
gradually  replaces  the  kernels  as  it  absorbs  the  food  stored  in 
them.  Later  the  infected  kernels  grow  to  many  times  the  size 
of  the  original  kernels  and  produce  great  masses  of  spores  in 
cavities  which  resemble  somewhat  those  already  described  in 


THE  FUNGI 


277 


the  puffball.  Finally  all  of  the  tissues  of  the  original  kernel 
are  absorbed  and  the  hyphse  die,  leaving  the  spores  surrounded 
by  a  thin  hyphal  membrane.  The  rupture  of  this  membrane 

allows  the  spores  to  escape  and  to  be 
widely  disseminated  in  the  fields. 
Like  the  black  spores  of  the  wheat 
rust  these  smut  spores  carry  the  plant 
over  the  winter  and  germinate  in  the 
spring,  producing  the  smaller  spores, 
or  sporidia,  which  are  blown  to  young 
corn  plants  and  start  the  fungus  for 
another  season. 

Other  smuts — 
on  onions,  wheat, 
oats,  etc. — have  a 
history  similar  to 
that  of  the  corn 
smut.  The  smuts 
are  very  destruc- 
tive where  they 
occur  in  abun- 
dance, since  they 
attack  the  ker- 
nels of  the  grains 
affected  and  thus 
cause  the  com- 
plete destruction 
of  the  seed. 


FIG.  154.     Kernels  of   corn 

infected     with     corn    smut 

(Ustilago  zeae) 

Observe  the  outline  of  the  ker- 
nels still  visible  in  the  upper, 
greatly   swollen  mass.     After 
Duggar 


FIG.  155.  Spore  germi- 
nation in  the  spores  of 
corn  smut  ( Ustilago  zeae) 

Note  the  conidia,  or  dis- 
seminating and  infecting 
spores,  being  produced 
from  the  sides  of  the  sin- 
gle germ  tube,  or  hypha, 
springing  from  the  spore. 
After  Brefeld 


LICHENS 

Structure  and  habit.  Lichens  occur  on  the  bark  of  trees,  on 
old  fences,  and  even  on  earth  and  stones  in  certain  localities. 
They  are  grayish  green  in  color  and  may  occur  as  flat,  leaflike 
expansions  (foliaceous  lichens)  (Fig.  156),  as  powdery  crusts 
(crustaceous  lichens),  or  as  shrubby,  erect  outgrowths  (fruticose 
lichens).  The  fruticose  lichens  often  form  conspicuous  coverings 


FIG.  156.   Habit  and  structure  of  a  common  tree  lichen,  Physcia  stellaris 

A,  habit  of  plants  bearing  reproductive  cups,  or  apothecia;  B,  section  of  a  reproduc- 
tive cup,  showing  the  dark  spore-bearing  layer  (hymenium)  lining  the  cup ;  C,  struc- 
ture of  upper  portion  of  plant  body,  including  the  green  algal  cells ;  D,  portion  of  the 
hymenial  layer,  greatly  magnified,  showing  spore  sacs  (asci),  with  dark-colored  spores, 
and  the  lighter  paraphyses.  From  Bergen  and  Davis's  "  Principles  of  Botany  " 


THE  FUNGI 


279 


of  branches  in  warm,  moist  climates,  where  they  are  popularly 
known  as  gray  moss,  since  they  resemble  mosses  very  closely. 
The  grayish-green  color  of  lichens  is  due  to  the  fact  that  the 
plant  body  is  composed  of  two  distinct  plants :  namely,  a  green 
alga  resembling  Protococcus  and  a  colorless  fungus  like  the 
molds.  These  two  plants  are  associated  as  partners  in  the  same 
plant  body,  the  alga  furnishing  the  sugar  made  by  photosynthesis 
in  its  green  cells, 
and  the  fungus  ab- 
sorbing the  water 
and  soil  salts  neces- 
sary for  the  growth 
of  •  each  partner. 
The  physical  rela- 
tions of  the  two 
plants  are  such  as 
to  enable  them  to 
maintain  their  part- 
nership to  the  best 
advantage.  The  flat, 
leaflike  body  of  a 
common  foliaceous 
lichen  is  composed 
largely  of  molcllike 
fungus  hyphse,  not 
unlike  a  very  dense 
and  regularly  formed  mycelium.  The  hyphse  on  the  upper  and 
lower  surfaces  of  the  flat  expansion  are  short  and  closely  ad- 
herent (Fig.  156,  (7),  thus  forming  the  protective  outer  layers 
termed  respectively  the  upper  and  lower  cortex.  Between  these 
two  layers  the  hyphse  form  a  looser  structure  termed  the  medulla, 
or  central  part  of  the  lichen  body.  The  algae  are  commonly  dis- 
tributed in  a  layer  just  beneath  the  upper  cortex,  where  they 
are  most  advantageously  exposed  to  light.  The  fungus  hyphse 
either  penetrate  the  algal  cells  by  means  of  short  branches 
which  absorb  food  from  the  vacuole,  or  they  adhere  closely  to 
the  algse  and  absorb  nutriment  through  their  cell  wall. 


FIG.  157.    A  common  lichen,  Parmelia,  on  the  bark 
of  a  hickory  tree 

The  spore-bearing  cups  are  clearly  shown 


280  GENERAL  BOTANY 

The  relation  of  the  fungus  to  the  algae  is  therefore  that  of  a 
parasite  on  a  green  plant,  while  the  algae  probably  derive  some 
benefit  from  the  fungus  in  raw  food  materials  and  protection. 
Such  a  relationship  is  called  symbiosis,  which  means  a  partnership 
with  mutual  advantage  to  each  partner.  This  physical  relation 
of  the  algae  and  the  fungus  of  the  lichen  body  is  easily  demon- 
strated by  making  sections  or  by  teasing  out  small  portions  of 
an  ordinary  lichen  in  a  drop  of  water.  The  striking  contrast 
between  the  bright-green  algal  cells  and  the  colorless  hyphae  of 
the  fungus  enables  one  to  determine  without  difficulty  in  such 
preparations  the  relation  and  the  nature  of  the  two  plant  partners. 

Reproduction.  Almost  any  part  of  the  plant  body  of  a  lichen, 
if  removed  from  the  mother  plant  and  placed  under  favorable 
conditions,  can  reproduce  the  plant  vegetatively.  The  usual 
method  of  reproduction,  however,  is  by  means  of  asexual  spores, 
which  are  formed  in  special  club-shaped  cells  termed  asci.  These 
spore-bearing  cells  occur  in  great  numbers  in  the  so-called  lichen 
cups,  which  are  easily  seen  covering  the  upper  surface  of  lichens 
in  fruit.  The  inner  surface  of  these  cups,  or  apothecia,  is  usually 
dark  brown  or  gray  in  color,  but  in  some  instances  it  may  be 
orange  or  brick  red.  A  section  through  the  cup  (Fig.  156,  #) 
shows  that  its  entire  inner  surface  is  lined  with  the  club-shaped 
spore-bearing  cells,  or  asci,  alternating  with  greatly  elongated 
sterile  hyphae  called  paraphyses.  Each  club-shaped  spore-bearing 
cell  contains  from  two  to  eight  spores,  which  are  expelled  with 
some  force  from  the  ascus  when  they  are  ripe.  These,  spores  ger- 
minate like  mold  spores,  and  if  the  hyphae  thus  formed  come 
in  contact  with  the  proper  algae,  they  attach  themselves  to  the 
algal  cells  and  gradually  form  a  new  lichen  plant.  It  is  thus 
seen  that  the  production  of  a  new  lichen  is  entirely  dependent 
upon  spore  reproduction  by  the  fungus.  In  this  manner  lichens 
arise  in  nature  wherever  rough,  moist  surfaces  offer  the  proper 
conditions  for  their  growth.  On  account  of  their  great  hardiness 
and  their  ability  to  make  their  own  food  the  lichens  are  among 
the  most  widely  distributed  plants  in  nature. 


THE  FUNGI  281 

FUNGI  AND  PLANT  DISEASE 

Nature  and  importance.  The  diseases  of  plants  caused  by 
fungi  are  scarcely  less  important  than  the  diseases  of  animals 
and  man,  on  account  of  their  intimate  relation  to  the  world's 
food  and  lumber  supply.  These  facts  were  abundantly  empha- 
sized to  the  public  during  the  Great  War,  when  government 
and  state  experts  demonstrated  the  immense  importance  of 
fungous  pests  to  our  national  food  supply.  We  have  only  to 
recall  the  damage  done  by  such  fungi  as  the  grain  rusts  and 
smuts,  the  potato  rot,  the  apple  scab,  the  grape  mildews,  and 
the  tree-killing  fungi  to  realize  the  national  importance  of  fungi 
and  fungous  diseases. 

Some  plant  diseases  are  produced  by  bacteria,  as  in  man  and 
animals,  but  the  greater  number  are  caused  by  filamentary  fungi 
similar  to  the  molds,  rusts,  and  smuts  studied  in  previous  pages. 

Spread  of  plant  diseases.  The  rapid  spread  of  plant  diseases 
is  due  in  large  measure  to  the  very  unusual  number  of  asexual 
spores  produced  by  the  fungi,  of  which  we  have  had  illustrations 
in  the  molds,  smuts,  and  mushrooms.  Indeed,  many  fungi,  like 
the  mushrooms  and  smuts,  have  apparently  abandoned  the  sexual 
method  of  reproduction  entirely  for  the  more  rapid  method  of 
producing  abundant  asexual  spores. 

These  spores  are  light  and  so,  like  the  bacteria,  are  constantly 
blown  about  and  carried  long  distances  by  air  currents.  As  a 
consequence  some  fungous  diseases  spread  with  startling  rapidity, 
resulting  in  widespread  injury. 

The  recent  spread  of  the  disease  known  as  chestnut  blight, 
caused  by  the  fungus  Endothia  parasitica,  is  a  well-known  illus- 
tration of  the  rapid  spread  of  such  a  destructive  disease  by  means 
of  spores.  The  chestnut  blight  is  supposed  to  have  been  imported 
from  China  and  to  have  spread  from  New  York  City,  as  a  center, 
about  1904.  It  has  now  almost  wholly  destroyed  the  native  chest- 
nut trees  over  large  areas  in  the  forests  and  cities  of  the  eastern 
United  States.  The  pine-tree  blister  rust  has  had  a  similar  his- 
tory and  threatens  to  destroy  many  thousand  feet  of  valuable 
pine  timber  unless  its  ravages  can  be  checked. 


282  GENERAL  BOTANY 

The  dissemination  of  fungi  by  spores  lodged  on  seeds  or  by 
means  of  mycelium  within  the  seeds  is  another  method  of  spread- 
ing fungous  diseases.  The  rusts  and  smuts  are  often  thus  dis- 
seminated by  spores  lodged  on  the  kernels  of  the  grain  when  the 
seeds  are  sown.  These  spores  then  germinate  with  the  seeds  and 
attack  the  young  seedlings  while  their  tissues  are  tender  and 
allow  the  fungus  hyphse  to  penetrate  them. 

Beans  are  infested  with  a  fungous  disease  known  as  anthracnose, 
which  often  does  great  damage  to  the  crop.  The  mycelium  of  the 
fungus  causing  the  disease  grows  into  the  ovule  during  its  active 
vegetative  life  and  hibernates  there  during  the  resting  period  of 
the  seed.  The  next  spring,  when  the  beans  are  planted,  these 
hibernating  hyphse  also  grow  and  form  a  mycelium  throughout 
the  tissues  of  the  bean  plant.  If  conditions  are  right  for  vigo- 
rous fungus  growth,  the  mycelium  attacks  the  pods  and  develop- 
ing seeds  and  destroys  the  crop. 

In  this  manner  a  dangerous  disease  may  readily  be  spread 
through  the  exchange  or  sale  of  seed,  and  the  infection  of  a  pre- 
vious year  may  cause  the  real  damage  to  the  crop  in  the  year 
following  infection.  Local  spreading  of  fungous  diseases  is  often 
caused  in  forests  by  certain  fungi  which  attack  the  roots  of  trees. 
These  fungi  form  long  stolons,  composed  of  fungus  hyphse, 
which  grow  for  considerable  distances  and  penetrate  the  roots  of 
adjacent  trees,  infecting  them  with  the  disease. 

In  addition,  dissemination  of  fungi  is  effected  by  water,  as 
in  the  water  molds  (Saprolegnici),  .by  animals  (including  insects 
and  fish),  and  by  man  in  the  shipping  and  transportation  of  plants 
and  plant  parts  from  state  to  state  and  from  one  country  to 
another.  The  spread  of  disease  by  this  method  has  been  greatly 
increased  in  recent  years  by  the  shipping  of  seeds  and  entire 
plants  from  state  to  state  and  from  European  countries  into 
the  United  States. 

Infection.  Plants  have  few  openings  into  the  inner  tissues 
of  the  body,  corresponding  to  the  nose,  mouth,  and  ears  of  man 
and  other  animals,  through  which  germs  can  enter  and  cause 
disease.  Infection  in  plants,  therefore,  is  more  likely  to  take 
place  through  wounds  or  through  the  stomata. 


THE  FUNGI  283 

Infection  of  forest  trees  (Fig.  151),  for  instance,  usually 
takes  place  at  points  where  wounds  have  been  made  by  prun- 
ing or  where  the  bark  has  been  injured  by  animals.  In  a  similar 
manner  the  infection  of  fruits  is  usually  caused  by  bruising, 
which  breaks  the  epidermis  and  thus  offers  an  opening  to  fungi, 
whose  spores  germinate  in  the  wound.  It  is  for  this  reason  that 
shippers  of  apples  and  other  fruits  exercise  the  greatest  care  in 
preparing  the  fruit  for  shipment.  This  is  done  by  drying  and 
rubbing  the  surface  of  the  fruit,  by  discarding  infected  speci- 
mens, and  by  wrapping  each  fruit  in  paper  to  prevent  contact 
and  infection  from  adjacent  specimens.  Many  of  these  fruits 
are  -infected  and  rotted  in  part  by  molds  like  Penicillium 
(Fig.  143),  the  spores  of  which  are  on  the  skin  of  the  fruit 
when  it  is  packed.  In  other  instances  fungi  infect  host  plants 
by  boring  through  epidermal  walls  through  the  agency  of  fer- 
ments, which  destroy  the  tissues  ahead  of  the  entering  germ 
tube.  Infection  by  a  filamentary  fungus  is  often  accompanied 
by  infections  on  the  part  of  bacteria  also,  which  complete  the 
processes  of  decay  inaugurated  by  the  higher  fungous  parasite. 

Invasion  and  disease  production.  The  fungus  hyphae,  when 
they  have  once  penetrated  into  the  interior  tissues  of  a  plant, 
spread  and  destroy  the  tissues  by  the  growth  of  a  mycelium, 
exactly  as  in  the  case  of  mold  on  bread  (Fig.  140)  or  like  a 
tree  parasite  (Fig.  152).  The  invasion  and  breaking  down  of 
the  tissues  is  effected  through  the  secretion  of  ferments  by  the 
hyphse  of  the  invading  mycelium,  just  as  the  hyphae  of  Rhizopus 
digest  the  starch  in  a  slice  of  bread  by  the  secretion  of  digestive 
ferments.  In  this  manner  the  mycelium  of  a  parasitic  fungus 
may  penetrate  the  hardest  wood  of  trees,  and  then  dissolve  out 
the  wall  substance  and  the  stored  food  by  a  digestive  process 
which  results  in  decay  and  death.  Smuts  and  similar  fungi, 
which  start  to  grow  with  the  seed,  keep  pace  in  the  above 
manner  with  the  growth  of  the  seedling,  following  along  the 
paths  of  the  least  resistant  tissues  to  places  where  the  spores 
are  to  be  formed. 

This  invading  of  the  plant  body,  and  the  production  of  disease, 
is,  however,  a  much  slower  process  in  plants  than  the  distribution 


284  GENERAL  BOTANY 

of  bacteria  and  their  poisonous  toxins  by  means  of  the  blood 
stream  in  animals.  As  a  consequence,  the  period  of  disease 
production  is  greatly  prolonged  in  plants  as  compared  with 
animals,  and  may  continue  in  perennials,  like  trees,  for  many 
years  without  causing  death. 

The  exact  manner  in  which  fungi  destroy  plant  structures  and 
materials  which  are  commercially  important  to  man  varies  with 
the  fungus  and  the  host.  In  the  case  of  forest  trees  the  damage 
comes  from  the  breaking  down  of  the  tissues  of  the  tree,  so  that 
the  wood  is  injured,  as  well  as  from  the  checking  of  the  healthy 
growth  and  development  of  the  tree.  In  herbaceous  plants  the 
fungous  disease  usually  weakens  the  plant  to  such  an  extent 
that  its  productiveness  is  greatly  lessened,  or  it  may  destroy 
the  plant  entirely.  In  other  cases,  as  in  the  attack  of  grain  by 
smuts,  the  spores  are  formed  in  such  numbers  in  the  ovules 
that  the  grain  itself  is  almost  entirely  destroyed.  In  soft  plant 
structures,  like  fruits  and  potatoes,  the  mold  or  other  parasite 
simply  starts  the  process  of  decay,  which  is  then  carried  on  by 
bacteria  until  the  entire  structure  is  destroyed. 

Remedies.  The  remedy  for  the  great  losses  in  food,  lumber, 
and  nursery  stock,  which  result  from  fungous  diseases,  lies  in  a 
better  knowledge  of  the  nature  of  these  diseases  and  in  a  more 
strict  oversight,  on  the  part  of  individuals  and  of  state  and 
national  governments,  of  their  control  and  dissemination.  The 
United  States  government  now  employs  expert  plant  pathologists, 
who  study  the  origin  and  spread  of  plant  diseases  throughout 
the  country.  They  are  also  cooperating  with  state  governments, 
through  the  state  experiment  stations,  in  an  endeavor  to  check 
the  progress  of  dangerous  fungous  pests.  Expert  pathologists 
are  also  maintained  by  the  United  States  government  to  inspect 
imported  seeds  and  plants,  so  as  to  prevent  the  introduction  of 
plant  diseases  from  foreign  countries.  Entire  shipments  of  fruits, 
vegetables,  and  nursery  stock  are  often  condemned  by  these 
government  officials  and  denied  entrance  into  our  ports.  Much 
insight  is  also  being  gained  into  the  various  methods  of  control- 
ling various  diseases  by  sterilizing  seeds  before  planting  and  by 
the  destruction  of  certain  fungous  hosts  which  harbor  dangerous 


THE  FUNGI 


285 


parasites.  The  order  of  the  government  to  destroy  the  bar- 
berry bushes  during  the  late  war  is  an  instance  in  point.  These 
barberries  were  known  to  be  infected  by  one  phase  of  the  wheat 
rust,  which  winters  on  wheat  straw  or  stubble.  By  destroying 
the  barberry  the  government  hoped  to  eradicate  this  dangerous 
pest.  Recent  orders  of  the  government,  looking  toward  the  con- 
trol of  shipments  of  bulbs  and  nursery  stock  from  Holland  and 


INCOME 


I.  FROM  SUN 
ENERGVv      ' 

2.  FROM  AIR     V 

CO*  \.          £ 


OUTGO 


NON-GREEN   ORGANISMS 


GREEN  PLANT 


FIG.  158.   The  organic  food  cycle 

other  European  countries,  are  interesting  indications  of  the 
awakening  of  the  country  to  the  great  danger  and  commercial 
importance  of  fungous  diseases  to  our  ornamental  and  food  plants. 


THE  ORGANIC  FOOD  CYCLE 

From  what  has  been  learned  concerning  the  life  of  the  fungi 
the  student  is  prepared  to  understand  more  clearly  the  relative 
position  of  the  fungi  and  the  green  plants  as  regards  the  food 
supply  of  all  organic  life  (Fig.  158).  We  have  seen  that  the 


286  GENERAL  BOTANY 

green  plants,  on  account  of  their  chloroplasts,  are  able  to  con- 
vert the  simple  elements  of  the  air  and  the  soil  into  organic 
food.  For  this  purpose  they  are  able  to  draw  on  the  energy  of 
the  sun  through  the  agency  of  their  chlorophyll  pigment.  Such 
plants  are  thus  able  to  secure  an  unlimited  supply  of  outside 
energy,  which  makes  them  the  great  food  makers  and  accumu- 
lators for  the  rest  of  organic  nature.  The  colorless  fungi  and 
the  animals  are  dependent  upon  the  green  plants  for  organic  food 
and  are  therefore  food  destroyers  and  energy  producers  in  the 
organic  world.  Many  of  the  green  plants  also  serve  directly  as 
food  for  animals  and  for  parasitic  fungi  of  various  sorts,  but 
other  green  plants  are  not  edible,  or  they  die  before  they  are 
devoured  by  animals  or  by  plant  parasites.  The  food  stored  in 
the  cell  walls  and  in  the  special  storage  organs  of  such  plants 
would  be  lost  to  the  living  organic  world  if  it  were  not  for  the 
saprophytic  fungi.  These  saprophytes,  as  we  have  learned,  are 
able  to  secrete  ferments  which  decompose  by  fermentation  or 
digestion  the  tissues  of  all  lifeless  organisms,  and  the  result  of 
such  decomposition  is  that  all  lifeless  organic  matter  is  ulti- 
mately reconverted  into  the  gases  of  the  air  and  the  salts  of  the 
soil  from  which  green  plants  obtain  their  raw  food  materials. 
There  is,  therefore,  a  continuous  cycle  taking  place  in  the  world's 
food  supply,  in  which  the  green  plants,  the  fungi,  and  the  animals 
are  mutually  interdependent  and  necessary  factors.  In  this  cycle 
green  plants  are  the  great  food  producers  and  energy  storers ; 
animals  and  parasitic  fungi  are  food  users  and  energy  pro- 
ducers; and  the  saprophytic  fungi  are  the  great  scavengers 
and  reconverters  of  lifeless  organic  matter  into  new  compounds, 
which  can  be  used  over  again  by  the  food-building  green  plants. 
It  is  difficult  to  see  how  any  one  of  the  members  of  this  triple 
alliance  of  organic  forms  could  exist  for  long  without  the  pres- 
ence of  the  other  two  members  of  the  organic  world.  When  we 
take  this  larger  view  of  nature,  therefore,  we  see  that  the  fungi 
are,  on  the  whole,  useful  plants. 


CHAPTER  XIY 

BRYOPHYTES  (LIVERWORTS  AND  MOSSES) 

The  Bryophyta  are  higher  types  of  plants  than  the  Thallophyta, 
approaching  more  nearly  to  the  conditions  found  in  the  higher 
spore  and  seed  plants  in  the  structure  of  the  plant  body  and  in 
the  character  of  the  reproductive  organs.  In  the  liverworts,  or 
Hepaticae,  the  plant  body  is  termed  a  thallus,  since  they  resemble 
the  thallop'hytes  (algse  and  fungi)  in  not  possessing  true  roots, 
stems,  and  leaves.  The  mosses  (musci)  are  more  highly  organized 
than  the  liverworts  and  have  stem  and  leaves  resembling  some- 
what those  of  the  higher  plants,  but  this  leafy-stemmed  plant  is 
a  gametophyte  plant,  as  in  the  algse,  instead  of  a  sporophyte 
plant,  like  the  leafy  plants  of  all  plants  above  the  bryophytes. 
The  liverworts  and  mosses,  while  differing  in  the  form  and 
structure  of  the  plant  body,  are  nevertheless  closely  related  by 
the  great  similarity  of  their  reproductive  organs  and  the  stages 
of  their  life  histories. 

HEPATICAE  (LIVERWORTS) 

The  simplest  members  of  the  Hepaticae  are  amphibious  plants, 
which  occupy  wet  banks  and  overhanging  rocks  on  the  borders 
of  streams  and  lakes.  They  are  named  Hepaticae  from  their 
fancied  resemblance  to  the  lobes  of  the  human  liver.  As  might 
be  expected,  these  simplest  land  plants  exhibit  transition  stages 
between  the  aquatic  algae  and  the  highest  spore  plants  (repre- 
sented by  the  mosses  and  ferns).  The  simplest  forms  are  mere 
ribbons  of  green  cells,  resembling  in  their  structure  the  leaf  of 
Elodea  or  of  a  moss.  They  are  attached  to  the  mud  or  wet  rocks 
over  which  they  grow  by  fine,  hairlike  roots  called  rhizoids, 
which  closely  resemble  the  root  hairs  of  the  higher  plants. 

287 


288  GENERAL  BOTANY 

Among  the  higher  liverworts  a  more  bulky  plant  body  enables 
these  plants  to  live  in  less  moist  situations  than  the  lower  types 
and  to  approximate  more  nearly  in  their  form  and  structure  to 
true  land  plants.  These  higher  forms  closely  resemble  in  struc- 
ture a  leaf  of  the  higher  plants  and  are  attached,  like  the  simpler 
species,  by  means  of  rhizoid  roots. 

The  production  of  new  plants  and  the  wider  distribution  of 
the  species  take  place  by  means  of  nonmotile  spores  borne  in 
special  spore  cases,  or  sporangia,  which  are  new  structures  in  the 
life  history  of  plants  thus  far  studied. 

RICCIOCARPUS 

Habit  and  habitat.  Ricciocarpus  (Fig.  159)  is  a  hydrophytic 
liverwort  which  floats  on  the  surface  of  ponds  and  lakes  or  grows 
on  mud  along  the  shore  where  the  water  has  receded. 

The  general  structure  of  the  plant  body  is  similar  to  a  green 
leaf  in  its  organization,  with  green  chlorophyll  tissue  on  its 
upper  exposed  surface  adapted  for  photosynthesis.  The  lower 
surface  is  furnished  with  rhizoids,  arid  with  platelike  structures 
in  the  form  of  scale  leaves,  which  serve  in  part  to  keep  the  plant 
upright  .on  the  surface  of  the  water.  In  the  case  of  plants  growing 
on  mud  the  rhizoids  function  as  roots  in  the  absorption  of  water 
and  soil  salts. 

Gametophyte.  The  plant  body  of  Ricciocarpus  is  the  gameto- 
phyte,  which  bears  the  reproductive  organs  in  furrows  on  the 
upper  surface,  back  of  the  growing  points.  The  reproductive 
organs,  which  we  have  called  gametangia  in  the  algae,  are  here 
more  highly  organized  and  hence  have  other  names  applied  to 
them.  The  male  reproductive  organ  is  called  the  antheridium 
and  is  composed  of  an  outer  layer  of  cells,  called  wall  cells, 
which  inclose  and  protect  the  inner  mass  of  mother  cells  of  the 
sperms,  or  male  gametes  (Fig.  159,  #).  The  protoplasts  of  these 
mother  cells  differentiate  to  form  the  male  gametes,  and  when 
the  gametes  are  fully  formed,  the  walls  separating  the  mother 
cells  are  absorbed  and  the  gametes  are  liberated  by  the  rupture 
of  the  antheridium  at  its  apex. 


BRYOPHYTES 


289 


The  female  reproductive  organ  is  also  more  highly  organized 
than  similar  organs  in  the  algae  and  is  called  an  archegonium 
(Fig.  159,  c,  c?).  This  archegonium  is  flask-shaped,  with  an  elon- 
gated neck  and  an  enlarged  venter.  The  outer  cells  of  the 
neck  and  venter  form  a  protective  wall  layer  like  that  of  the  an- 
theridium,  inclosing  a  central  column  of  cells  more  immediately 


Neck 


Yen 


Neck-canal 
—  cells 
---Wall  cells 
Ventral- 
canal  cell 


/-^^v*«  "      ^ 
a        \ 
Archegonial  furrow     Antheridial  furrow 


Neck 

Archegoni 

Venter 

!-- $  Pronuclcus 
IPronucleus 
d 


Sperms 


Wall  cells 

Sperm 
mother 
cells 


FIG.  159.    Habit  and  sexual  reproductive  organs  of  Eicciocarpus 

a,  habit  of  a  single  plant  of  Ricciocarpiis ;  6,  section  of  the  plant  represented 
in  a  cut  through  two  reproductive  furrows  containing  archegonia  and  antheridia; 
c,  section  of  young  archegonium ;  d,  section  of  archegtraium  with  fertilized  egg 
cell  containing  male  (d)  and  female  (?)  pronuclei;  e,  young  antheridium;  /,  sperms 
with  flagella;  g,  section  of  mature  antheridium 

concerned  with  fertilization  and  the  growth  of  an  embryo.  This 
central  column  of  cells  includes  the  neck-canal  cells,  the  ventral- 
canal  cell,  and  the  female  gamete. 

Fertilization  takes  place  when  the  plants  are  wet  with  rain, 
dew,  or  spray,  since  this  is  the  proper  condition  for  liberating 
the  gametes  and  for  the  locomotion  of  the  motile  male  gametes. 
When  the  female  gamete  is  ready  for  fertilization,  the  neck-canal 
and  ventral-canal  cells  disorganize  and  form  a  mucilaginous 
substance  which  absorbs  water,  ruptures  the  archegonium  at  its 
apex,  and  exudes  in  the  form  of  a  viscid  drop  in  which  the 


290 


GENERAL  BOTANY 


liberated  male  gametes  become  entangled.  The  male .  gametes 
then  swim  down  the  canal  of  the  neck,  made  by  the  disorgani- 
zation of  the  canal  cells,  and  one  successful  gamete  enters  the 
egg  and  fertilizes  it.  The  unsuccessful  gametes,  as  in  the  algge 
and  fungi,  are  unable  to  penetrate  the  fertilized  egg  cell  and  die 
in  the  neck  canal  or  the  venter. 

Sporophyte.  The  zygote  germinates  at  once  without  passing 
through  a  resting  period  as  in  the  algse.  The  growth  of  the 
zygote  (Fig.  160)  is  accompanied  by  cell  division  and  differen- 
tiation as  in  the  zygote  of  the  mandrake  or  the  bean.  The  embryo 


\ 


Venter  cell 

Wall  cells 

Spore 
mother  eel 


FIG.  160.   Development  of  spores  (sporogenesis)  in  Eicciocarpus 

a,  early  spore  mother-cell  stage,  with  mother  cells  forming  a  cellular  tissue ;  6,  the 

mother  cells,  after  becoming  free,  have  each  divided  twice  and   formed  four  cells, 

called  a  tetrad ;  c,  the  four  cells  of  each  tetrad  in  6  separating  to  form  four  spores 

which  results  is,  however,  very  simple  in  Ricciocarpus  as  com- 
pared with  the  embryos  of  the  higher  plants  just  referred  to.  It 
is  composed  of  an  outer  layer  of  wall  cells  which  incloses  and 
protects  a  mass  of  cells  which  ultimately  become  the  mother 
cells  of  spores.  With  the  enlargement  of  the  embryo  the  spore 
mother  cells  become  free  from  each  other,  and  each  mother  cell 
divides  twice  to  form  groups  of  cells  in  fours,  called  tetrads. 
Each  cell  of  a  tetrad  gives  rise  to  a  spore.  By  this  process  the 
embryo  is  converted  into  a  spore  case,  or  sporangium,  filled  with 
spores.  The  rupture  of  the  wall  of  this  spore  case  -liberates  the 
spores,  which  then  germinate  at  once  to  form  new  plants  in  a 
proper  habitat. 

Such  a  plant  body  as  that  described  above,  which  results 
from  cell  division  and  differentiation  of  the  zygote,  is  called  a 
sporophyte,  since  its  main  function  is  the  bearing  of  spores. 


BRYOPHYTES 


291 


We  shall  learn,  as  indicated  above,  that  this  simple  sporophyte 
of  the  liverwort  is  the  forerunner  of  the  leafy  green  plant  body 
of  the  ferns  and  the  seed  plants  like  the  mandrake  and  the  bean. 

The  sporophyte  of  Ricciocarpus  is  therefore  a  simple  kind  of 
sporophyte  plant,  produced  from  the  zygote,  which  grows  as 
a  parasite  on  the  mother  plant,  or  gametophyte,  and  is  devoted 
wholly  to  the  production  of  spores. 

Life  history.  In  Fig.  161  the  main  stages  in  the  life  history 
of  Ricciocarpus  and  Vaucheria  are  represented  by  diagrammatic 
figures  to  bring  out  the  essential  points  of  contrast  between  them. 

Sex     Gametes       Sporo- 
Gametopliyte    organs     \    Zygote  phyte    Spore         Gametophyte 


Gametophyte  Sex     "~T 

organs  Gametes 


Gametophyte 


FIG.  161.  Life  history  of  Ricciocarpus  and  of  Vaucheria,  represented  graphically 
a,  stages  in  the  life  history  of  Ricciocarpus ;  b,  stages  in  the  life  history  of  Vaucheria 

In  the  liverwort  the  gametophyte  is  bulkier  and  more  highly 
differentiated  than  in  the  alga,  and  is  thus  better  able  to  meet  the 
aquatic  or  amphibious  conditions  under  which  it  has  to  live. 

The  reproductive  organs,  instead  of  being,  as  in  the  alga, 
single-celled  structures  adapted  to  form  the  gametes,  are  com- 
plex cellular  organs,  with  an  outer  protective  cell  layer  inclosing 
the  more  delicate  cells  which  form  the  gametes  and  aid  in 
fertilization. 

The  zygote  is  formed  in  both  the  liverwort  and  the  alga  by 
the  union  of  the  male  and  female  gametes  in  fertilization,  and 
consists  in  each  case  of  a  single  cell  with  a  protective  cell  wall, 
cytoplasm,  and  nucleus,  like  the  zygote  of  Spirogyra. 


292 


GENERAL  BOTANY 


In  the  alga  this  cell  usually  passes  through  a  resting  period, 
after  which   it   germinates  to  form    a  new   filamentary  plant, 

or  gametophyte. 

In  Ricciocarpus  the 
zygote  becomes  an 
embryo  plantlet,  or 
sporophyte,  through 
cell  division  and  cell 
differentiation,  as  in 
the  higher  plants 
already  studied,  but 
in  the  embryo  of 
Ricciocarpus  cell  di- 
vision and  differen- 
tiation are  limited, 
and  so  the  resulting 
sporophyte  plant  is 
relatively  simple,  as 
we  have  seen. 

The  sporophyte 
of  Ricciocarpus  evi- 
dently corresponds 
in  the  life  history 
to  the  embryo  and 
the  adult  plant  of 
the  bean  or  the  man- 
drake, since  they  are 
all  derived  by  simi- 
lar processes  from 
an  initial  fertilized 
egg  cell,  or  zygote. 
Plants  like  Riccio- 
carpus and  the  man- 
drake, which  have  a  sporophyte  plant  alternating  in  the  life 
history  with  the  gametophyte  plant,  are  said  by  botanists  to 
have  an  alternation  of  generations  in  the  life  cycle. 


FIG.  162.   Habit  of  a  moss,  showing  the  leafy  game- 
tophytes  and  sporophytes.   After  Sachs 


BRYOPHYTES  293 

MUSCI  (MOSSES) 

Habit.  The  mosses  are  leafy-stemmed  plants  and  are  hence 
much  more  highly  organized  than  the  liverworts,  which  are  their 
nearest  relatives.  The  stem  of  such  a  moss  as  Funaria  is  a 
delicate  structure  with  the  leaves  arranged  spirally  upon  it. 
The  leaves  are  also  very  simple,  being  composed  of  a  single 
layer  of  chlorophyll-bearing  cells,  except  in  the  central  conduct- 
ing strand,  corresponding  to  a  midrib,  where  the  cells  are 
elongated  and  are  two  or  more  layers  thick. 

The  plants  are  anchored  in  the  soil  by  delicate  rhizoids,  like 
those  of  liverworts,  which  serve  a  similar  function.  Funaria, 
like  most  mosses  and  liverworts,  grows  in  clusters,  —  a  habit 
which  is  an  advantage  in  conserving  moisture  and  in  insuring 
fertilization,  since  the  male  and  female  organs  are  borne  on 
separate  plants  and  the  clustering  habit  is  necessary  in  order  to 
insure  the  proximity  of  male  and  female  organs  (Fig.  163). 

FUNARIA 

Gametophyte.  The  leafy  moss  plant  just  described  is  the  game- 
tophyte  corresponding  to  the  'flat,  leaflike  plant  body  of  Riccio- 
carpus  and  the  other  liverworts.  In  Funaria  the  reproductive 
organs  (archegonia  and  antheridia)  are  borne  in  terminal  repro- 
ductive buds. 

The  antheridia  are  borne  in  the  form  of  open  disks  (Fig.  163,  <?), 
which  are  easily  recognized  by  the  brick-red  color  of  the  wall  cells. 

The  archegonia  (Fig.  163,6)  are  produced  in  closed  buds,  which 
are  not  easily  distinguished  from  vegetative  buds  (Fig.  163,  a). 

The  antheridia  and  archegonia  are  almost  identical  in  struc- 
ture and  function  with  those  already  described  in  Ricciocarpus, 
so  that  no  additional  description  of  them  is  necessary.  The 
antheridia  (Fig.  163,  6?,  e)  are  accompanied  by  multicellular 
hairs,  called  paraphyses,  with  enlarged  terminal  cells. 

Fertilization  takes  place,  as  in  the  liverworts,  when  the  plants 
are  wet  with  rain  or  dew,  which  facilitates  the  opening  of  the 
sex  organs  and  the  migration  of  the  motile  male  gametes. 


294 


GENERAL  BOTANY 


The  male  gametes  swarm  down  the  opened  neck  canal  of  the 
archegonium  as  in  Ricciocarpus,  and  the  egg  cell  is  fertilized  by 
one  successful  male  gamete.  The  zygote  which  results,  like  that 
of  RictiocarpviS,  forms  an  outer  cell  wall  and  begins  to  germinate 
without  passing  through  a  resting  period. 

Sporophyte.  The  embryo  (Fig.  164,  a)  which  results  from  the 
early  divisions  of  the  zygote  cell  resembles  somewhat  the  young 
embryo  sporophyte  of  Ricciocarpus.  Elongation  of  this  cellular 
embryo  then  begins,  resulting  in  a  rodlike  structure  which  carries 


Archegonia     Sperms. 


es  Anaieridium 


#• 


/ 


^c 
FIG.  163.    Male  and  female  plants  and  sex  organs  of  a  dioecious  moss,  Funaria 

a,  female  plant  with  reproductive  bud;  b,  archegonia  (left,  young  archegonium 
with  egg  and  canal  cells ;  right,  older  archegonium  with  egg  in  the  venter) ;  c,  male 
plant  with  group  of  antheridia  forming  antheridial  disk  at  apex  of  plant ;  d, 
group  of  antheridia  and  paraphyses;  e,  mature  antheridium  expelling  sperms; 
/,  sperms  with  flagella 

the  old  archegonium  wall  upon  its  apex  like  a  cap,  the  wall 
cells  having  separated  from  the  plant  at  the  base  of  the  venter 
(Fig.  164,  c  and  d).  This  rodlike  cellular  zygote  grows  not  only 
upward  but  downward  as  well,  so  that  it  bores  with  its  basal 
end  into  the  tissues  of  the  leafy  moss  plant,  from  which  it  now 
draws  its  food  and  water  like  a  parasite.  After  it  has  become 
established  in  the  leafy  gametophyte  plant,  the  rod  begins  to 
enlarge  in  its  upper  part  to  form  the  rudimentary  spore  case 
known  as  the  moss  capsule,  while  its  lower  part  is  converted  into 
a  strong,  flexible  rod  called  the  seta.  This  spore  case,  or  capsule, 
although  at  first  composed  of  simple,  undifferentiated  cells,  soon 
becomes  differentiated  into  a  complex  cellular  organ  for  the  pro- 
duction of  spores.  In  many  mosses  the  outer  cell  layers  of  the 


BBYOPHYTES 


295 


capsule  beneath  the  epidermis  are  green  and  are  therefore  able 
to  carry  on  some  photosynthesis,  but  the  greater  part  of  the  food 
required  for  the  development  of  the  spores  within  the  capsule 
is  derived  from  the  mother  plant  through  the  seta. 

When  the  spores  are  ripened,  the  inner  tissues  of  the  capsule 
break  down  and  the  spores  lie  free  within  its  central  cavity. 


-Seta 


FIG.  164.    Stages  in  the  development  of  the  sporophyte  in  Funaria 

a,  archegonium  containing  the  cellular  embryo  of  a  young  sporophyte ;  b,  moss  plant 
with  rodlike  emhryo  still  in  the  expanded  archegonium ;  c,  rod  stage  of  the  sporo- 
phyte with  the  old  archegonium  wall,  neck,  and  venter ;  d,  mature  sporophyte  with 
parts  differentiated  ;  e,  parts  of  the  capsule ;  /,  portion  of  the  peristome ;  g,  gameto- 
phyte  and  sporophyte  in  proper  relation 

The  mature  capsule  at  this  time  consists  of  the  following  struc- 
tures, represented  in  Fig.  164,  e  and  /.  A  lid,  or  operculum,  is 
released  at  the  apex  of  the  capsule,  thus  exposing  the  light  spores 
to  the  air.  When  the  lid  is  removed,  a  circle  of  hairlike  teeth,  the 
peristome,  is  seen  to  guard  the  mouth  of  the  capsule.  These  teeth 
have  the  power  of  movement  and  tend  to  close  over  the  mouth 
of  the  capsule  'in  wet  weather  and  to  open  out  in  dry  weather, 
when  the  light  spores  are  more  easily  scattered  by  the  wind.  In 
many  mosses  the  teeth  open  by  chinks  in  dry  weather  only,  so 
that  the  spores  are  sifted  out,  like  salt  out  of  a  salt  shaker, 


296 


GENERAL  BOTANY 


when  the  elastic  seta  allows  the  capsule  to  be  swayed  by  the  wind. 
We  see,  therefore,  how  admirably  the  moss  capsule  is  organized 
to  form  and  disseminate  the  spores  which  give  rise  to  new  moss 

plants.  These  spores  are  highly 
protected  cells,  not  unlike  the  zy- 
gotes  of  Spirogyra  in  structure, 
although  their  asexual  method  of 
origin  is  quite  different  from  the 
latter,  as  we  have  seen. 

Under  the  proper  conditions  of 
temperature  and  moisture  the  spores 
germinate  and  form  a  mass  of  fila- 
ments, which  often  coat  wet  banks 
or  flowerpots  with  a  green  coating 
resembling  closely  some  of  the  algse. 
From  these  filaments  new  leafy 
moss  plants  arise  (Fig.  165),  which 
in  turn  form  gametes  and  start  the 
cycle  of  a  new  generation. 

Life  history.  The  life  history  of 
the  moss  is  identical  with  that  of 
Ricciocarpus  except  in  the  greater 
complexity  of  the  gametophyte  and 
sporophyte  plants  and  in  the  details 
of  spore  germination.  In  both  cases  the  gametophyte  and  sporo- 
phyte alternate  in  regular  sequence  in  the  life  history,  the  game- 
tophyte plant  doing  the  work  of  photosynthesis  and  the  bearing 
of  the  reproductive  organs,  while  the  sporophyte  lives  parasiti- 
cally  on  the  gametophyte  and  produces  the  spores. 


FIG.  165.  Protonema  and  young 
moss  plants 

Young  plant  of  a  moss  attached 

to  a  protonema  with  reproductive 

buds  (6) 


CHAPTER  XV 

PTERIDOPHYTES  (FERNS,  EQUISETA,  AND  CLUB  MOSSES) 

The  Pteridophyta  are  a  very  interesting  and  important  division 
of  the  plant  kingdom,  not  only  on  account  of  their  great  beauty 
and  common  occurrence  but  also  because  they  are  the  undoubted 
ancestors  of  the  higher  seed  plants.  These  fernlike  ancestors  of 


FIG.  166.   Maidenhair  ferns  (Adiantum)  growing  in  a  natural  habitat 
in  the  forest 

the  pines  and  their  allies,  as  well  as  of  the  flowering  plants  (like 
the  beans,  mandrake,  and  clovers),  date  back  to  the  coal  period. 
The  special  advances  made  by  the  Pteridophyta  over  the 
plants  below  them  relate  to  the  much  greater  importance  of 
the  sporophyte  as  compared  with  the  gametophyte  in  the  life 
histories  of  all  its  members,  and  to  the  differentiation  of  the 


298 


GENERAL  BOTANY 


spores  into  two  distinct  kinds,  which  has  been  effected  by  some 
of  the  higher  living  species  and  fossil  species  of  the  coal  age. 
These  special  features  of  their  life  histories  indicate  clearly 
the  important  part  played  by  the  early  ancestors  of  modern 
ferns  in  the  origin  of  the  living  seed  plants.  These  important 
features  in  the  evolution  of  the  plant  body  and  reproduction 
of  the  Pteridophyta  will  be  more  clearly  understood  after  con- 
sidering the  life  history  of 
type  species  selected  from 
the  three  great  orders  of  liv- 
ing Pteridophyta,  namely,  the 
true  ferns,  the  lycopods,  and 
the  horsetails. 


FI LI  GALES  (FERNS) 

Habitat  and  habit.  The 
ferns  are  quite  unlike  the 
mosses  and  liverworts  in  their 
structure  and  mode  of  life, 
since  they  are  true  land  plants 
with  an  organization  similar 
to  that  of  the  higher  seed 
plants.  The  great  majority 
of  ferns  inhabit  the  floor  of 
forests,  where  the  light  is 
dimmed  by  overhanging  foli- 
age and  the  soil  is  moist  and  rich  with  decaying  humus.  A  few 
forms  only  are  adapted  to  xerophytic  conditions,  and  fewer 
still  are  hydrophytes.  Most  ferns  are  therefore  typical  medium 
land  plants,  or  mesophytes,  adapted  to  the  conditions  of  light, 
soil,  and  air  found  on  the  forest  floor  (Fig.  166).  The  leaves 
are  large -and  divided  to  form  the  characteristic  compound  leaf 
seen  in  most  ferns.  The  leaf  blades  are  thin,  and  the  mesophyll 
cells  possess  an  abundance  of  chlorophyll,  which  is  favorable  to 
the  manufacture  of  starch  and  sugar  in  a  dim  and  moist  atmos- 
phere ;  the  stem  in  most  instances  is  an  underground  rhizome 


FIG.  167.  Coiled  tips  (circinate  vernation) 
of  a  fern,  Osmunda  claytoniana 


PTERIDOPHYTES 


299 


like  that  already  described  for  some  herbaceous  seed  plants,  and 

the  young  leaves  spring  from  the  apex  of  the  stem  and  uncoil 

in  the  peculiar  circinate  manner  shown  in  Fig.  167.    They  are 

often  protected  in  the  early  stages  of  their  development  by  the 

abundant  scales,  or 

ramenta  (singular, 

r amentum),  which 

are    so    character- 

istic  of  ferns.  The 

roots  (Fig.  168)  are 

true  roots,  like  those 

of  seed  plants,  and 

not    hairlike    rhi- 

zoids  like  those  of 

the  moss  group. 

The  cell  struc- 
ture and  the  tissues 
of  the  stem,  roots, 
and  leaves  of  ferns 
are  also  very  simi- 
lar to  those  of  the 
higher  plants.  In- 
deed, ferns  are  the 
first  of  the  vascular 
plants  which  possess 
a  true  vascular  conducting  system  composed  of  phloem  and 
xylem.  For  these  various  reasons  they  are  now  regarded  as  the 
ancestors  of  higher  seed  plants. 


FIG.  168.  Habit  and  parts  of  a  single  plant  of  Adiantum 
a,  the  entire  plant ;  b,  portion  of  a  pinna 


SPOKOPHYTE 
ADIANTUM  (MAIDENHAIR  FERN) 

Habit.  The  maidenhair  fern  is  like  the  other  members  of  the 
Filicales,  mentioned  above,  in  having  an  underground  stem,  or 
rhizome  (Fig.  168),  which  is  thickly  covered  with  ramenta.  These 
scales  are  particularly  abundant  at  the  growing  point  and  on  the 
young  leaves,  and  serve  as  a  protective  covering. 


300 


GENERAL  BOTANY 


Pith  (staring) 

Vascular  strand  (conducting) 
Cortex  A  (storing) 


Meristem 


Leaf 


The  young  leaves  unfold  in  the  spring  in  the  circulate  manner 
represented  in  Fig.  168  and  finally  develop  a  dark,  shining, 
elastic  leafstalk,  which  forks  repeatedly  to  form  the  compound 
leaf.  The  mature  leaf  is  pinnate  (like  a  feather),  and  the  ultimate 
green  divisions  are  called  pinnules.  The  veins  of  the  pinnules 
are  forked,  or  dichotomous,  like  the  veins  of  other  ferns,  and  the 
edges  of  the  pinnules  are  reflexed  to  cover  the  sporangia  which 
produce  the  spores. 

The  seasonal  life  of  Adiantum  corresponds  exactly  to  that  of 
perennial  herbaceous  seed  plants,  like  the  mandrake,  which  have 

an  underground  rhizome  and 
-Leaf  trace  annual    aerial    parts.     The 

leaves  manufacture  food  by 
photosynthesis  ;  some  of  the 
food  is  used  for  the  season's 
growth,  while  the  excess 
passes  down  the  leaf  and 
into  the  rhizome,  where  it 
is  stored  for  use  the  next 
spring.  When  frost  comes, 
the  leaves  of  the  maidenhair 
die  down  and  the  rhizome 
hibernates  and  carries  the 
plant  over  the  winter.  With 

the  advent  of  spring  the  reserve  food  is  digested,  the  young 
leaves  begin  to  uncoil,  and  another  active  period  in  the  life  of 
the  plant  is  begun.  In  order  to  carry  on  these  various  functions 
the  tissues  of  the  fern  stem  and  leaf  are  more  highly  differentiated 
than  those  of  any  plants  yet  studied  except  the  seed  plants. 

Structure.  The  general  arrangement  of  the  main  tissues  of 
the  rhizome  of  Adiantum  is  diagrammatically  represented  in 
Fig.  169.  In  this  figure  it  may  be  noted  that  the  tissues  of  the 
fern  stem  are  the  same  in  kind  and  general  arrangement  as  that 
already  studied  in  herbaceous  dicotyledons.  The  outside  of  the 
stem  is  protected  by  an  epidermis  and  by  the  thick-walled  cells 
of  the  outer  cortex,  which  constitute  a  kind  of  ezoskeleton  for 
the  entire  stem.  The  inner  portions  of  the  cortex  and  pith  are 


FIG.  169.    Diagram  of  long  section  of  the 

rhizome,  showing  the  main  tissue  areas 

and  their  principal  functions 


PTERIDOPHYTES 


301 


composed  of  storage  parenchyma,  in  which  the  reserve  food  is 
stored  for  the  spring  growth.  The  vascular  system  is  composed  of 
a  phloem  and  xylem  cylinder  which  runs  lengthwise  of  the  stem. 
From  this  vascular  cylinder  strands  called  leaf  traces  pass  out 
through  the  cortex  into  the  leaves.  Where  a  leaf  trace  passes 
off  from  the  vascular  cylinder  a  break  called  a  leaf  gap  occurs 
(Fig.  169).  The  roots  also  connect  with  the  central  vascular  cylin.- 
der,  so  that  a  continuous  vascular  system  exists  in  the  maidenhair 


-^Skeletal  tissue 
— Storage  tissue  \ 
—Phloem^          \ N 
— Xylem 


tfortex 

idermis 


Pith 

-Epidermis 
Cortex 


"Leaf  trace 1- 


~Lcaf  trace     / 
^Leaf  petiole^      / 
^^Leaf  gap 

FIG.  170.   Gross  anatomy  of  a  portion  of  the  rhizome  of  Adiantum 

a  shows  the  relation  of  the  vascular  system  to  the  other  tissues  and  to  the  leaves 
(note  the  breaks  or  leaf  gaps  in  the  vascular  cylinder,  caused  by  the  exit  of  the  leaf- 
trace  bundle ;  the  cortex  has  been  removed  from  one  side  to  expose  the  wood  cylinder, 
leaf  gaps,  and  leaf  traces  in  surface  view) ;  b,  transverse  section  of  the  rhizome  cut  in 
the  region  of  a  leaf  gap  and  a  leaf  trace 

and  other  ferns  as  in  the  higher  plants.  The  arrangement  of  the 
tissues  in  a  transverse  and  a  longitudinal  section  of  the  rhizome  is 
shown  in  Fig.  170.  Fig.  170,  £,  shows  the  concentric  arrange- 
ment of  phloem  and  xylem  which  is  characteristic  of  fern  stems. 
In  this  arrangement  the  phloem  completely  incloses  the  xylem, 
flanking  it  on  the  outside  next  to  the  cortex  and  on  the  inside 
next  to  the  pith.  In  the  seed  plants,  including  the  alder  and  Salvia 
(Fig.  56),  it  will  be  remembered  that  the  arrangement  was  col- 
lateral-, that  is,  with  the  xylem  on  the  inside  of  the  vascular 
cylinder  next  to  the  pith,  and  with  the  phloem  on  the  outside 
next  to  the  cortex.  The  concentric  arrangement  of  phloem  and 


302 


GENERAL  BOTANY 


xylem  in  the  ferns  is  a  very  ancient  one  in  vascular  plants  and 
gave  rise  to  the  collateral  arrangement  by  a  gradual  process  of 
evolution,  the  discussion  of  which  is  not  within  the  scope  of  an 
elementary  textbook:  The  cambium  is  entirely  lacking  in  the 
stem  of  the  maidenhair  and  in  the  stems  of  most  living  ferns, 

so  that  no  secondary  phloem,  xylem, 
or  wood  rays  are  developed.  Storage 
is  therefore  largely  limited,  in  all  liv- 
ing FilicaleSj  to  the  wide  cortex  and 
the  large  pith. 

The  general  form  of  the  vascular 
cylinder  in  the  maidenhair,  and  the 
relation  of  the  leaf  traces  and  leaf 
gaps  to  it,  are  matters  of  considera- 
ble interest  and  importance,  since 
they  aid  in  the  correct  interpretation 
of  similar  structures  in  the  higher 
plants  and  also  give  the  student  a 
clearer  conception  of  the  continuity 
of  the  vascular  system  and  of  the 
gross  anatomy  of  fern  stems. 

Fig.  1 70,  a,  shows  a  portion  of  the 

FIG.  171.   Plant  of  Pteris  aqui-       ,  .  , ,  -_c    -i     •         i  •  i 

llna  (the  eagle  fern),  showing     rhlzome  grea%  magnified,  in  which 
rhizome  and  leaf  the  outer  cortex  has  been  removed 

The  rootstock  (rh)  is  horizontal  from  one  side  so  as  to  expose  the 
and  grows  underground;  upon  it  centrai  vascular  cylinder  composed 

are  the  buds  (6)  and  the  upright  j 

leafstalk  (*«)  of  xylem   and  phloem.    By  an  in- 

spection of   this   figure    it   may  be 

seen  that  a  portion  of  the  phloem  and  xylem  ring  of  the  cen- 
tral cylinder  passes  out  to  form  the  vascular  bundle  of  each 
leaf  petiole.  This  vascular  bundle,  or  leaf  trace,  thus  connects 
the  phloem  and  xylem  of  the  main  vascular  ring  with  the  leaf 
veins.  The  figure  shows  also  that  the  entire  vascular  cylinder 
of  the  fern  rhizome  is  not  a  completely  closed  cylindrical  tube, 
but  is  broken  at  intervals  by  leaf  gaps.  In  fern  rhizomes 
where  the  leaves  are  large  and  close  together  the  gaps  are 
more  frequent,  and  the  vascular  cylinder  comes  to  resemble  a 


PTERIDOPHYTES 


303 


latticework  or  meshwork   with  vascular   strands   composed   of 
phloem  and  xylem  surrounding  the  large  and  frequent  leaf  gaps. 
In  Fig.  170,  ft,  the  relation  of  the  leaf  trace  and  the  leaf  gap 
is  shown  as  it  would  appear  in  a  transverse  section. 


STRUCTURE  OF  PTEEIS  (BRACKEN  FERN) 

In  the  bracken  fern  (Pteris  aquilina)  the  vascular  system  is 
more  complicated  than  that  of  the  maidenhair  fern.  Instead  of 
a  nearly  complete  vascular  cylinder  of  phloem  and  xylem,  broken 
by  infrequent  short 
leaf  gaps,  as  in  the 
maidenhair,  the  vas- 
cular cylinder  of 
Pteris  is  broken 
by  many  long  leaf 
gaps  into  a  set  of 
vascular  strands, 
or  bundles,  which 
form  a  coarse  mesh- 
work,  with  infre- 
quent union  of  the 
strands  composing 
the  mesh.  This 
meshwork,  when 
cut  across,  forms 
the  outer  circle  of 
vascular  bundles 

seen  in  Fig.  172.  The  two  inner  bundles,  flanked  on  either  side 
by  dark  masses  of  skeletal  supporting  tissues,  are  formed  by  a 
budding  process  from  the  outer  bundle  cylinder  in  the  early 
stages  of  the  development  of  the  stem.  In  Fig.  173,  a,  the  sec- 
tion of  the  young  stem  of  Pteris  is  seen  to  correspond  almost 
exactly  with  that  of  Adiantum.  In  ft,  £,  and  d  the  budding  off 
of  a  central  bundle  and  the  division  of  the  single  bud  to  form 
the  two  central  bundles  of  the  adult  stem  are  diagrammatically 
shown.  The  origin  of  the  two  dark  masses  of  skeletal  tissue  from 


FIG.  172.    Section  of  the  rhizome  of  Pteris  aquilina 

.9,  internal  skeletal  tissue  composed  of  thick-walled  fibers ; 

fb,  vascular  strands  of  the   meshwork  comprising  the 

vascular  cylinder  of  Pteris 


304 


GENERAL  BOTANY 


Skeletal  strands 

Bad 

(liriilcil 


a  single  strand,  and  the  conversion  of  the  solid  vascular  bundles 
by  an  increasing  number  of  leaf  gaps,  is  also  shown  in  the  figures. 
At  the  left  in  c  and  d  a  root  is  being  given  off. 

The  minute  structure  of  the  tissues  of  ferns  is  strikingly 
shown  in  the  large-celled  tissues  of  Pteris.  Each  vascular 
bundle  is  concentric,  as  in  the  maidenhair,  with  a  central  xylem 

mass   surrounded  by   a 
sheath  of  phloem. 

In  long  sections  the 
phloem  is  seen  to  be 
composed  of  large  sieve 
tubes  having  numerous 
sieve  plates,  together 
with  considerable  phloem 
parenchyma  (Fig.  174). 
The  xylem  is  made  up 
of  single-celled  water- 
conducting  elements,  or 
tracheids,  which  have  the 
wall  markings  of  ducts. 
These  tracheids  in  Pteris 
and  some  other  ferns 
often  unite  end  to  end 
to  form  primitive  water 
ducts.  Such  a  primitive 
duct,  with  a  ladderlike 
(scalariform)  arrangement  of  the  thick  and  thin  places  of  the 
cell  wall,  is  shown  in  Fig.  174.  A  large  sieve  tube,  with  innu- 
merable sieve  plates  of  varying  form  and  size,  is  also  shown. 
Both  the  duct  and  the  sieve  tube  are  flanked  by  living  storage 
parenchyma  cells.  The  tissues  of  the  cortex  and  epidermis  are 
similar  in  arrangement  and  structure  to  those  of  the  maiden- 
hair. The  cell  walls  of  the  epidermis  and  skeletal  tissues  are 
greatly  thickened,  and  the  cells  of  the  latter  are  elongated  and 
are  fibrous  in  their  nature.  These  two  tissues,  together  witli 
the  inner  skeletal  strands,  form  an  admirable  protective  and 
strengthening  exoskeleton  for  the  fern  rhizome. 


FIG.  173.  Diagram  illustrating  the  development 
of  the  stem  tissues  in  Pteris  aquilina 

a,  section  through  a  young  rhizome ;  b,  section 
through  an  older  portion  of  a  rhizome,  showing  the 
hud  which  forms  the  two  internal  bundles;  c,  d, 
older  stages  in  the  development  of  the  internal  bun- 
dles and  the  skeletal  tissue.  After  Jeffrey.  Re- 
drawn from  Jeffrey's  "  Anatomy  of  Woody  Plants  " 


PTERIDOPHYTES 


305 


Pericycle 
\  Cortex 


SUMMARY 

As  regards  their  vegetative  structures  the  ferns  may  be  character- 
ized as  follows : 

1.  They  are  the  simplest  vascular  plants  and  have  become  adapted 
to  a  moist  mesophy tic  environment  by  the  formation  of  large,  thin, 
dark-green  leaves. 

2.  They  are  really  herba- 
ceous perennials,  with  a  sea- 
sonal life  quite  comparable 
to  that  of  many  seed  plants 
with  a  perennial  underground 
stem.     The  tree  ferns  of  the 
present  day  and  those  of  the 
coal  period  are  exceptions  to 
this  general  herbaceous  habit. 

3.  A  few  ferns  are  adapted 
to  dry  conditions,  and  some 
are  hydrophytes. 

4.  The    anatomy    of    the 
fern  rhizome,  or   stem,   has 
the  same  general  tissue  ar- 
rangements as  the  herbaceous 
stems   of  the  higher  flower- 
ing plants,  with  a  large  pith, 
a  wide  cortex,  and  a  narrow 
wood    ring.     The   distinctive 
features  are  the  conspicuous 
leaf  gaps  (which  often  trans- 


Duct  Parenchyma  [  Endodermis 
Parenchyma  Sieve  tube 

FIG.  174.  Vascular  tissues  of  the  rhizome 
of  Pteris  in  long  section 


After  Jeffrey.   Redrawn  from  Jeffrey's 
"  Anatomy  of  Woody  Plants  " 


form  the  vascular  cylinder  into  a  meshwork  instead  of  an  unbroken 
cylinder)  and  the  concentric  arrangement  of  phloem  and  xylem. 
There  is  no  cambium  layer  and  consequently  no  increase  in  diam- 
eter by  the  production  of  annual  rings  of  phloem  and  xylem.  The 
water-conducting  elements  are  more  primitive  in  type  than  those  of 
the  higher  seed  plants  studied  earlier  in  the  course. 

ASEXUAL   REPRODUCTION 

The  leaves  of  ferns  bear  the  asexual  spores  in  definite  organs, 
called  sporangia,  located  on  the  lower  side  of  the  leaves  along 
the  veins.  These  sporangia  are  borne  in  groups  called  sori 


306 


GENERAL  BOTANY 


(Fig.  176,  A,  B,  (7)  and  are  covered  in  the  maidenhair  by  the 
reflexed  margins  of  the  pinnules  (Fig.  168,  b).  Each  sporan- 
gium (Fig.  175)  is  composed  of  a  stalk  and  an  enlarged  spore 
case,  or  sporangium  proper.  This  spore  case  is  composed  of  an 
outer  layer  of  cells  and  an  inner  mass  of  cells  which  ultimately 
form  the  spores.  A  mature  sporangium  is  also  furnished  with 
a  ring  of  thick-walled  cells,  the  annulus,  which  brings  about  the 
final  rupture  of  the  sporangium  and  the  scattering  of  the  spores. 
This  process  is  effected  as  follows :  The  cells  of  the  annulus  are 
filled  with  water  during  the  early  stages  in  the  development 


FIG.  175.    Sporangia  of  a  fern,  showing  the  development  and 
dissemination  of  spores 

a,  sporangia  containing  young  and  mature  spores ;  b,  c,  opening  of  the  sporangia  at 
the  lip  cells ;  d,  spore  dissemination  hy  the  movement  of  the  annulus 

of  the  sporangium.  As  the  sporangium  matures  and  the  spores 
ripen,  this  water  begins  to  evaporate,  and  the  thin  outer  walls  of 
the  annulus  cells  are  pulled  in  and  become  U-shaped.  They 
are  supposed  to  be  pulled  in  by  the  cohesive  strength  of  the 
diminishing  water  in  their  cell  cavities.  As  the  outer  walls  be- 
come concave  the  entire  annulus  bends  backward  and  ruptures 
the  wall  of  the  sporangium  at  its  weakest  point,  called  the  stoma. 
Finally,  when  the  water  is  nearly  withdrawn  from  the  cells,  its 
tension  breaks,  and  their  overstretched,  thick  inner  and  radial 
walls  snap  the  annulus  back  to  its  original  position  (Fig.  175,  d). 
This  snap  throws  the  spores  so  that  they  can  be  disseminated 
by  wind  and  air  currents.  These  spores  are  highly  protected 
cells  which  have  the  power  of  withstanding  drought  and  other 


PTERIDOPHYTES 


307 


FIG.  176.    A  fern  (Aspidium  Filix-mas)  bearing  sporangia  and  spores 

A,  rhizome  and  leaves ;  fr,  young  leaves ;  B,  pinnule  with  sori  (s) ;  C,  vertical  section 
of  a  portion  of  a  pinnule  (p),  showing  indusinm  (i)  and  sporangia  (s) ;  D,  single 
sporangium.  After  Wossidlo.  From  Bergen  and  Davis's  "  Principles'  of  Botany" 

inclement  conditions.  Bower  estimates  that  a  large  fern  plant 
like  Aspidium  (Fig.  176)  may  produce  as  many  as  ffty  million 
spores  in  a  season,  which  are  scattered  by  the  above  device  to 
perpetuate  the  particular  fern  by  which  they  are  produced. 


308 


GENERAL  BOTANY 


The  spores,  in  a  suitably  moist  and  warm  location,  produce  a 
little  plant,  the  gametophyte,  which  bears  the  sex  organs,  as  in 
the  gametophyte  plants  of  the  algse  and  the  mosses. 


GAMETOPHYTE  AND  EMBRYO 

Habit.  The  gametophyte  produced  by  the  germination  of  the 
spores  is  a  green,  heart-shaped  body  resembling  a  moss  leaf  in 

its  general  struc- 
ture (Fig.  177,  <?). 
The  entire  plant 
is  not  more  than 
one  fourth  of  an 

Gamctophyle  •       i        •          i 

inch  in  length 
and  is  furnished 
with  rhizoids  on 
its  under  surface, 
which  anchor  it 
to  the  moist  soil 
on  which  it  grows. 
The  margins  of 
the  little  plant 
are  composed  of 
a  single  layer  of 
cells,  but  the 
central  portion  is 
several  cells  in 
thickness.  This 
thick  portion  is 
called  the  cushion  and  bears  the  reproductive  organs  and  the 
rhizoids.  It  is  thus  equipped  like  the  moss  for  carrying  on  its 
own  nutrition  and  for » producing  the  sex  organs  necessary  for 
sexual  reproduction. 

Sexual  reproduction.  The  reproductive  organs  (Fig.  177)  re- 
semble those  of  the  liverwort  and  moss  so  closely  that  it  will  not 
be  necessary  to  describe  them  in  detail.  The  archegonia  are  borne 
at  the  anterior  end  of  the  cushion,  while  the  antlieridia  are  more 


cell 


Neck  canal  cells 


FIG.  177.    Spore  germination,  gametophyte,  and  sex 
organs  in  ferns 

a,  b,  two  stages  in  spore  germination;    c,  mature  gameto- 
phyte  with    antheridia   and    archegonia;     d,    mature    an- 
theridium  with  sperms;    e,   sperms  with  cilia;   /,  young 
archegonium ;  g,  mature  archegonium 


PTEKIDOPHYTES 


309 


or  less  spherical  structures  distributed  among  the  rhizoids  at  the 
posterior  end  of  the  plant.  This  position  of  the  sex  organs  next 
to  the  moist  soil  is  therefore  favorable  to  fertilization  \)j  the  mo- 
tile male  gametes  of  the  fern.  Fertilization  is  always  preceded, 
as  in  the  moss,  by  the  expulsion  of  the  male  gametes  during 
wet  weather  or  when  the  plants  are  covered  with  dew,  and  by 
the  transformation  of  the  canal  cells  of  the  archegonium  into  a 
mucilaginous  substance  which  attracts  the  sperms  to  the  eggs. 


A 


Rhizome 
c 


FIG.  178.    Gametophyte  and  embryo  of  Adiantum 

A,  garaetophyte  and  young  attached  sporophyte  of  Adiantum ;  B,  section  of  gameto- 
phyte  and  embryo  of  a  fern :  a,  young  embryo ;  6,  more  mature  embryo  with  first 
leaf ;  c,  permanent  rhizome,  leaves,  and  roots,  which  grow  from  the  stem  portion  of 
the  embryo  in  b.  The  embryonic  root,  foot,  and  leaf  of  the  embryo  in  b  disappear 
when  the  plant  in  c  is  fully  established 

Embryo.  After  fertilization,  while  still  in  the  venter  of 
the  archegonium,  the  zygote  germinates  to  form  the  embryo 
(Fig.  178,  B,  a).  The  young  sporophyte,  quite  unlike  that  of 
the  moss,  very  early  differentiates  into  a  primary  root,  stem, 
and  cotyledon,  which  indicates  that  it  is  to  become  a  real  leafy 
plant  with  the  usual  organs  of  the  higher  plants  (Fig.  178  5,  6). 
In  addition  to  these  primary  organs  the  embryo  spore  plant 
develops  a  mass  of  cells  called  the  foot,  which  remains  in  con- 
tact with  the  gametophyte  plant  and  absorbs  nourishment  from 
it  for  the  growing  embryo  until  the  latter  is  self  supporting  (c) 
and  the  gametophyte  dies. 


310  GENERAL  BOTANY 

The  adult  fern  plant,  which  was  described  above,  is  produced 
entirely  from  the  stem  portion  of  the  embryo  sporophyte.  The 
relation  of  the  adult  plant  to  the  embryo  is  diagrammatically 
illustrated  in  Fig.  178,  £,  6,  c,  in  which  the  embryonic  stem  is  rep- 
resented as  having  elongated  into  the  horizontal  leaf-and- 
root-bearing  stem  of  the  adult  fern.  In  the  figure  the  primary 
structures  —  root,  foot,  and  first  leaf  —  are  unshaded,  while  the 
perennial  adult  fern  plant  is  left  shaded.  This  perennial  fern 
plant,  therefore,  corresponds  to  the  moss  capsule  in  the  life  cycle 
of  the  two  organisms,  since  both  originate  from  the  zygote  and 
both  have  for  a  main  function  the  production  of  asexual  spores. 

Life  history.  The  essential  stages  in  the  life  history  of  ferns 
are  the  same  as  those  of  the  mosses,  in  that  the  zygote  develops 
in  each  case  a  sporophyte  which  alternates  with  a  gametophyte  in 
the  life  history.  The  spore  is  also  in  each  case  the  means  by 
which  the  mosses  and  ferns  multiply  and  disseminate  their  off- 
spring, and  it  furnishes,  therefore,  a  necessary  motile  stage  in  the 
life  history  of  these  stationary  land  plants. 

The  great  advance  made  by  the  fern  over  the  moss  is  in  the 
fact  that  the  sporophyte  in  the  fern  is  a  highly  organized  plant 
with  true  roots,  stem,  and  leaves,  in  which  highly  differentiated 
tissue  systems  provide  amply  for  food  making,  food  conduction, 
and  food  storage.  The  fern  sporophyte  is  therefore  an  independ- 
ent food-making  and  spore-producing  plant,  quite  different  from 
the  leafless  and  rootless  parasitic  sporophyte  of  the  moss.  The 
gametophyte,  on  the  contrary,  has  become  greatly  reduced  in  the 
fern  and  has  for  its  sole  functions  the  production  of  gametes  and 
the  temporary  support  of  the  embryo  sporophyte  (Fig.  178,  A). 
The  gametophyte  is  therefore  short-lived  and  usually  dies  at  the 
close  of  a  single  season's  growth.  We  shall  see  that  this  reduction 
of  the  gametophyte  to  a  comparatively  simple  plant  structure 
marks  the  beginning  of  the  almost  complete  disappearance  of  this 
phase  of  the  life  history  in  the  highest  seed  plants.  Since  the  ferns 
are  the  undoubted  ancestors  of  the  early  seed  plants  of  the  coal 
period,  the  facts  of  their  life  history  are  of  the  utmost  interest 
and  importance  to  a  proper  understanding  of  the  conditions 
which  we  are  to  discuss  later  in  those  plants. 


PTEKIDOPHYTES  311 

Chromosome  reduction.  Reduction  of  the  chromosomes  takes 
place  in  ferns  in  the  first  divisions  of  the  mother  cells,  so  that 
the  spores  have  one  half  as  many  chromosomes  in  their  nuclei 
as  the  nuclei  of  the  mother  cells  (consult  Fig.  43,  5).  The  nuclear 
division  which  follows  to  form  cells  of  the  gametophyte  and  the 
gametes  is  of  the  ordinary  vegetative  type  (Fig.  43,  a),  in  which 
each  daughter  nucleus  is  supplied  with  the  same  number  of  chro- 
mosomes as  the  nucleus  from  which  it  arose.  As  a  result  all 
cells  of  the  gametophyte,  including  the  gametes,  have  the  re- 
duced number  of  chromosomes.  When  fertilization  takes  place, 
the  nucleus  of  the  zygote  has  the  two  sets  of  chromosomes  sup- 
plied by  the  male  and  female  gametes  (consult  Fig.  44),  and 
this  double  number  persists  throughout  all  cellular  divisions  in 
the  sporophyte  plant  until  the  mother  cells  of  the  spores  divide 
to  form  the  first  two  cells  of  the  tetrads.  The  two  alternating 
generations  in  the  life  history  of  the  ferns  are  therefore  dis- 
tinct as  regards  the  number  of  chromosomes  in  the  nuclei  of  their 
constituent  cells,  the  gametophyte  and  the  gametes  always  having 
one  half  the  number  of  chromosomes  of  the  zygote,  the  embryo, 
and  the  adult  sporophyte,  as  in  the  higher  plants  (Fig.  44). 
The  chromosome  changes  in  the  life  history,  as  noted  above, 
always  take  place  in  the  first  division  of  a  spore  mother  cell  to 
form  tetrads  and  in  the  formation  of  the  double  cell,  or  zygote, 
which  results  from  the  union  of  the  gametes.  The  great  differ- 
ences in  the  form  and  structure  of  the  two  alternating  generations 
in  the  fern  are  therefore  paralleled  by  a  striking  difference  in  the 
chromosome  number  in  each  generation. 


LIFE  HISTORIES  OF  THE  FERN  ALLIES  (EQUISETALES 
AND  LYCOPODIALES) 

The  spore  plants  of  the  Uquisetales  and  Lycopodiales  differ 
so  much  from  the  true  ferns  in  external  appearance  and  struc- 
ture that  they  would  never  be  recognized  as  fern  allies  by  the 
ordinary  observer.  This  alliance  was  first  recognized  by  botanists 
after  Hofmeister,  in  1851,  had  worked  out  the  life  histories  of 
some  of  the  higher  spore  and  seed  plants  and  had  established 


312 


GENERAL  BOTANY 


FIG.  179.    Equisetum  arvense 

A,  plant  in  early  spring,  with  aerial  branch  (sb) 
bearing  strobili  (c)  and  a  leafy  branch  (fb) ;  B,  a 
green  summer  shoot;  C,  sporangiophore  with 
stalk  (st)  and  sporangia  (sp) ;  D,  spore  with  ex- 


the  fact  that  all  higher  green 
plants  had  similar  stages  and  a 
like  sequence  of  stages  in  their 
life  histories.  In  the  brief  discus- 
sion which  follows,  the  life  histo- 
ries of  the  equiseta  and  lycopods 
will  be  compared  with  that  of 
the  fern,  in  order  to  extend  the 
conceptions  that  have  been  gained 
with  regard  to  the  life  history  of 
ferns  and  of  the  fern  allies.  Sela- 
ginella,  with  its  two  kinds  of 
spores,  will  be  taken  as  a  rep- 
resentative of  the  Lycopodiales, 
in  order  to  introduce  the  stu- 
dent to  the  problems  connected 
with  the  origin  of  the  seed  and 
of  the  seed  habit  in  the  highest 
green  plants. 

EQUISETALES,  OR  HORSETAILS 
SPOROPHYTE 

Habit.  The  sporophyte  plants 
of  the  equiseta  differ  in  impor- 
tant particulars  from  those 
of  the  ferns  on  account 
of  their  xerophytic  form 
and  structure  (Fig.  179). 
Some  of  the  modern  equi- 
seta, like  a  few  of  the  ferns, 
inhabit  wet  marshes  and 


ponds,  but  they  all  retain 

panded  elaters ;  E,  spore  with  contracted  elaters  J 

the   xerophytic    structure 

and  appearance  which  is  derived  from  their  treelike  ancestors 
of  the  coal  period.  The  xerophytic  structures  of  the  equiseta 
are  therefore  genetic  and  not  adaptive  characteristics. 


PTERIDOPHYTES  313 

These  xerophytic  characteristics  are  apparent  in  the  minute, 
scalelike  leaves,  the  highly  protected  epidermis  of  the  stems, 
and  the  poorly  developed  vascular  system  in  the  horsetails, 
all  of  which  are  in  strong  contrast  to  the  large  leaves  and 
highly  organized  vascular  system  of  their  mesophytic  allies 
the  true  ferns.  The  plant  body  in  the  equiseta  is  composed  of 
an  underground  perennial  stem,  or  rhizome,  which  serves  the 
function  of  storing  food  and  of  hibernation  when  the  aerial 
parts  of  the  plant  are  destroyed  by  cold  or  drought.  This 
perennial  rhizome  gives  rise  to  annual  green  shoots,  which  are 
simple  in  some  species  and  highly  branched  in  others.  These 
aerial  stems,  since  they  produce  distinct  chlorophyll-bearing 
tissue,  serve  the  double  function  of  making  starch  by  photo- 
synthesis and  of  bearing  asexual  spores.  The  plant  body  thus 
manifests  the  same  physiological  division  of  labor  as  we  have 
already  noted  in  the  ferns,  but  the  work  of  photosynthesis  and 
spore  bearing  is  performed  in  the  equiseta  by  the  annual  shoots, 
instead  of  by  the  leaves  as  in  the  ferns. 

Asexual  reproduction.  The  spores  are  borne  in  strobili,  or 
cones,  which  terminate  either  the  regular  green  shoots  or  spe- 
cial reproductive  branches  as  in  Equisetum  arvense  (Fig.  179). 
The  reproductive  cones  (c)  consist  of  a  number  of  shield-shaped 
sporangiophores  ((7),  each  of  which  bears  several  sporangia,  shaped 
like  the  finger  of  a  glove.  When  the  spores  are  ripe,  the  axis 
of  the  cone,  or  strobilus,  lengthens,  thus  separating  the  spo- 
rangia, which  then  open  along  a  seamlike  layer  of  cells  on  one 
side  and  liberate  the  spores.  Each  spore  (.Z?)  is  furnished  with 
four  appendages,  called  elaters,  which  open  out  in  dry  weather 
and  close  up  around  the  spore  in  a  moist  atmosphere.  When 
spread  out,  the  elaters  assist  in  the  distribution  of  the  spores  by 
air  currents.  The  spores  germinate  much  as  in  the  ferns  and 
produce  a  gametopJiyte  plant,  which  bears  the  gametes.  The 
peculiarity  of  the  spores  of  Equisetum  in  this  regard  is  that  some 
spores  produce  gametophytes  which  bear  antheridia  only,  white 
others  produce  only  female  gametophytes.  The  spores  are  alike 
in  form  and  are  therefore  said  to  be  homosporous ;  but  they  are 
evidently  differentiated  physiologically,  since  they  produce  either  a 


314  GENERAL  BOTANY 

male  or  a  female  plant.  This  physiological  differentiation  of  spores 
in  Equisetum  is  very  important,  since  it  indicates  how  those  spore 
differences  probably  originated  which  gave  rise  to  the  so-called 
heterosporous  plants,  of  which  we  shall  learn  in  our  later  studies. 

GAMETOPHYTE  AND  EMBRYO 

The  male  and  female  gametophytes  of  the  equiseta  resemble 
those  of  the  ferns,  except  that  they  are  more  highly  branched 
(Fig.  180).  The  male  plants  are  small  and  bear  the  male  sex 
organs  at  the  ends  of  the  lobes  or  on  the  margins  of  the 

Sporophyte 

Spore  Gametophyte        '•.'(£&  "    Sporangia 

germination    Gametophytes       Sex  organs     Gamete*  Zygote    and  sporophyte  %  liji  and  spore 


Gametophyte  Sporophyte 

FIG.  180.    Diagram  showing  life  history  of  Equisetum 

gametophyte.  The  female  plants  are  larger  than  the  males  and 
bear  the  female  gametangia  on  a  thickened,  cushionlike  portion 
of  the  gametophyte  beneath  green,  lobelike  extensions  of  its 
upper  surface.  The  male  and  female  sex  organs  (.antkericUa  and 
archegonia)  and  the  gametes  indicate  by  their  structure  and  form 
their  close  relation  to  similar  reproductive  cells  and  organs  of 
the  ferns.  Fertilization  takes  place  by  means  of  motile  male 
gametes,  which  are  liberated  in  films  of  water  formed  by  rain 
or  dew.  Fertilization  of  the  female  gamete  is  followed  by  the 
development  of  a  parasitic  embryo  sporophyte,  as  in  the  ferns. 
Embryo.  The  general  form  and  structure  of  the  embryo 
sporophyte  in  the  equiseta  is  determined  by  the  kind  of  adult 
sporophyte  into  which  it  is  destined  to  develop.  We  find  as  a 


PTERIDOPHYTES  315 

consequence  that  the  equisetum  embryo,  on  account  of  the 
prominence  of  the  stem  in  the  adult  plant,  forms  as  its  promi- 
nent organs  a  rudimentary  stem,  root,  and  sucking  foot,  instead 
of  a  cotyledon,  root,  and  foot,  as  in  the  ferns.  This  embryo 
remains  parasitic  on  the  green  gametophyte  for  a  time,  but  ulti- 
mately gives  rise  to  the  adult  sporophyte  already  described. 
Life  history.  The  life  history  of  Equisetum  (Fig.  180)  will  be 
found  upon  inspection  to  present  the  same  sequence  of  stages  as 
that  of  the  fern,  together  with  a  close  resemblance  between  the 
two  forms  as  regards  the  structure  of  their  reproductive  organs 
and  cells.  These  facts,  as  stated  above,  establish  the  close  alliance 
of  the  equiseta  and  the  ferns  in  spite  of  the  great  dissimilarity 
in  the  form  and  structure  of  the  adult  sporophytes. 

LYCOPODIALES,  OR  CLUB  MOSSES 
SPOROPHYTE 

Habit.  In  the  Lycopodiales  the  sporophytes  are  often  mis- 
taken for  mosses  on  account  of  their  small  leaves  and  their 
habit  of  growing  in  clusters  or  mats  on  the  forest  floor  of  tem- 
perate and  tropical  regions.  Their  structure  and  life  history, 
however,  indicate  clearly  that  they  are  near  relatives  of  the 
ferns  and  equiseta  described  above.  The  Lycopodiales  are  divided 
into  two  closely  related  families,  the  lycopods  and  selaginellas, 
on  account  of  an  important  difference  in  the  method  of  produc- 
ing their  asexual  spores.  The  lycopods,  commonly  known  as 
ground  pines  (Fig.  181),  bear  but  one  kind  of  spores,  like  the 
ferns  and  equiseta,  but  the  selaginellas  (Fig.  182)  produce  two 
kinds  of  spores  in  different  sporangia  on  the  same  plant.  In 
other  respects  the  sporophytes  of  the  two  families  are  quite 
similar,  with  their  horizontal  branching  stems  creeping  over  the 
surface  of  the  ground  and  with  the  erect  spore-producing  shoots 
(strobili)  furnished  with  the  small  green  leaves  characteristic  of 
the  order.  The  entire  plant  body  is  anchored  to  the  ground  by 
naked  branches  sent  out  from  the  horizontal  stem ;  these  grow 
downward  and  take  root  in  the  soil  much  like  the  runners  of  a 
strawberry  plant. 


316 


GENERAL  BOTANY 


Asexual  reproduction.    The  erect  branches  terminate  in  the 
reproductive  strobili,  or  cones,  formed  of  modified  leaves,  which 

bear  the  sporangia  and 
spores.  These  very  highly 
modified  leaves  are  called 
sporop  hylls,  w  hich  me  an  s 
leaves  that  bear  spores. 
The  sporophylls  and  spo- 
rangia of  the  lycopods  and 
selaginellas  are  very  simi- 
lar except  that  in  Selagi- 
nella  there  are  two  kinds 
of  sporangia  and  two  kinds 
of  spores.  The  selaginel- 
las are  therefore  said  to 
be  heterosporous,  or  with 
different  spores,  to  dis- 
tinguish them  from  the 
common  ferns,  equiseta, 
and  lycopods,  which  are 
homosporous,  that  is,  with 
one  kind  of  spores.  It 
should  be  stated  also  that 
some  of  the  true  ferns 
now  living  are  heterospo- 
rous,  and  that  many  of  the 
fossil  Pteridophyta  of  the 
coal  period  bore  two  kinds 


of  spores.  Since  the  heter- 
osporous  condition  is  of 
the  greatest  importance 
for  an  adequate  under- 
standing of  the  life  his- 
tory of  seed  plants,  the 
following  description  of  the  sporangia  and  spores  of  Selaginella 
is  given  to  explain  the  nature  of  heterospory  in  a  common  tropi- 
cal species  of  the  Lycopodiales. 


FIG.  181.  The  habit  of  a  lycopod 
(Lycopodium) 

A,  plant  with  horizontal  rhizome  and  erect,  leafy 
spore-bearing  stem ;  str,  strobili ;  B,  spores ; 
C,  sporophyll  and  kidney -shaped  sporangium. 
From  Bergen  and  Caldwell's  "Practical  Botany" 


PTEEIDOPHYTES 


317 


Heterospory.  In  the  fertile  shoots  of  Selaginella  the  small  spores, 
called  microstores,  are  borne  in  microsporangia  at  the  apex  of  the 
reproductive  strobili,  while  the  large  spores,  called  megaspores, 
are  produced  in  large  megasporangia  at  the  base  of  the  strobili 
(Fig.  182).  The  microspores,  like  the  spores  of  the  equiseta  and 
thehomosporous  lycopods,  are  produced 
in  large  numbers  in  each  microsporan- 
gium,  but  the  large  megaspores  are 
reduced  to  from  two  to  four  spores  in 
each  of  the  megasporangia  (Fig.  183). 
This  discrepancy  in  size  is  closely  re- 
lated to  the  function  of  the  two  kinds 
of  spores  in  the  role  which  each  plays 
in  reproduction.  "  The  diameter  of  the 
megaspore  is  usually  about  ten  times 
that  of  the  microspores,  which  is  equiv- 
alent to  a  proportion  of  1000:1  in  bulk." 
The  larger  megaspores  produce  female 
gametophytes  and  on  account  of  their 
size  and  weight  are  not  easily  scat- 
tered by  the  wind  when  shed  from 
the  sporangium.  The  small  micro- 
spores,  on  the  contrary,  are  light  and  easily  scattered,  like  the 
pollen  grains  which  we  have  already  studied  in  the  higher 
plants.  Like  the  pollen,  therefore,  the  microspores  are  the 
mobile  spores  of  Selaginella,  which  can  be  borne  to  the  heavy 
megaspores  at  the  proper  time  for  reproductive  purposes. 
Since,  however,  this  association  of  megaspores  and  micro- 
spores  is  necessarily  a  matter  of  chance,  it  follows  that  the 
microspores  must  be  produced  in  great  numbers  to  insure  fer- 
tilization. Although  the  differentiation  of  spores  in  Selaginella 
and  in  other  heterosporous  PteridopTiyta  must  be  regarded  as 
the  first  step  toward  the  evolution  of  seeds,  it  is,  nevertheless, 
in  some  respects  a  wasteful  arrangement  and  not  without  its 
difficulties  in  insuring  a  meeting  of  the  male  and  female  gametes. 
In  the  higher  seed  plants  this  difficulty  is  overcome  in  part  by  the 
production  of  the  stigma  as  a  receptive  apparatus  for  the  pollen. 


Root 


FIG.  182.  Habit  of  Selaginella, 
with  strobili 


318 


GENERAL  BOTANY 


GAMETOPHYTES  AND  EMBRYO 

The  spores  of  Selaginella  begin  the  process  of  germination 
while  they  are  still  retained  in  the  sporangium  and  are  being 
nourished  by  the  mother  sporophyte.  The  germination  process 
differs  from  that  of  the  ferns  and  equiseta  also  in  that  the  entire 
process  takes  place  within  the  spore  coat  and  results,  therefore, 
in  rudimentary  gametophytes,  which  are  permanently  retained 

StroUlus  Megasporophylls       Strobilus         MicrosporopJiyllsi 

Sporangia  and' spores       axis         Sporangia  and  spores 


-  -MicrosporopJiyll 
i—Microsporangium 


FIG.  183.   Spore  production  in  Selaginella 

A,  strobilus  with  megasporophylls  and  megasporangia ;    B,  megasporophyll  with 

sporangium  and  spores;  C,  median  long  section  of  a  strobilus;  D,  microsporophyll 

with  sporangia  and  spores 

in  the  male  and  female  spores.  The  female  gametophyte  is  ulti- 
mately exposed  to  the  air  by  the  rupture  of  the  large  mega- 
spore  along  three  lines  of  weakness  in  the  spore  coat,  which  thus 
exposes  the  gametophyte  and  the  archegonia  in  preparation  for 
the  fertilization  process  (Fig.  184,  a,  6).  The  germinated  female 
spores  may  still  be  retained  in  the  sporangium  or  they  may  be 
shed  upon  the  earth,  where  the  archegonia  and  gametes  go 
through  the  same  preparatory  stages  for  receiving  the  male 
gametes  as  we  have  already  described  in  the  mosses  and  ferns. 
Meanwhile  the  rudimentary  male  gametophytes  are  developed 
inside  of  the  microspores ;  each  of  these  consists  essentially  of  a 
small  cell  representing  the  male  gametophyte,  and  of  an  anther- 
idium  composed  of  wall  cells  and  of  several  sperm  mother  cells 


PTERIDOPHYTES 


319 


which  finally  form  free  male  gametes  within  the  microspore 
(Fig.  184,  <?).  These  male  gametes  are  then  liberated  by  the  rup- 
ture of  the  spore  coat  and  fertilize  the  female  gametes.  In  some 
species  of  Selaginella  fertilization  and  the  early  development  of 
the  embryo  take  place  within  the  female  sporangium,  which  has 
previously  opened  and  admitted  the  germinated  microspores.  In 


d  e 

FIG.  184.    Gametophytes,  sex  organs,  and  embryo  of  Selaginella 

a,  megaspore  opening  to  expose  the  inclosed  gametophyte  and  archegonia  preparatory 
to  fertilization ;  6,  sectional  view  of  a  megaspore  and  gametophyte ;  c,  section  of  a 
microspore,  gametophyte,  and  sperm  mother  cells,  with  sperms  above;  d,  e,  three 
stages  in  the  development  of  the  sporophyte  of  Selaginella;  e,  young  and  differ- 
entiated sporophyte,  with  elongating  root  and  stem 

other  instances  fertilization  may  take  place  on  the  soil,  where 
the  female  spores  complete  the  germination  process.  The  process 
of  fertilization,  as  in  all  other  instances  among  the  higher  plants, 
produces  an  embryo  sporophyte  which  ultimately  grows  into  an 
adult  spore  plant  of  Selaginella. 

Embryo.  The  young  embryo  (Fig.  184,  d)  is  early  differentiated 
into  the  embryo  proper  and  the  suspensor,  which  serves  to  force 
the  embryo  down  into  the  nutritive  tissue  of  the  gametophyte. 
The  embryo  then  develops  two  cotyledons  and  a  rudimentary 


320 


GENERAL  BOTANY 


stem,  root,  and  foot  surrounded  by  food  material  previously  stored 
in  the  gametophyte  within  the  spore.  The  further  growth  of  the 
embryo  (e)  is  accomplished  by  the  elongation  of  the  young  stem 
(or  hypocotyl)  and  the  root,  which  bore  their  way  out  of  the 
gametophyte  and  become  adjusted  to  soil  and  air  in  response  to 
gravity  and  light. 

Life  history.  The  life  history  of  Selaginella  (Fig.  185)  pre- 
sents certain  new  and  important  features.  The  spores  are  of 
two  kinds:  small  microspores,  resembling  pollen  grains,  which 


Spores 


Spores 


Gametophyte  Sporophyte  Gametophyte 

FIG.  185.   Life  history  of  Selaginella  represented  graphically 

are  easily  carried  by  air  currents ;  and  large  megaspores,  which 
develop  a  nutritive  gametophyte  and  female  gametes.  Spore 
germination  takes  place  while  the  spores  are  still  in  the  spo- 
rangia, and  consequently  the  gametophyte  plants  thus  formed 
within  the  spore  coats  are  nourished  by  and  are  parasitic  upon 
the  mother  sporophyte.  The  male  gametophytes  are  reduced 
to  gamete-producing  structures  and  are  composed  of  a  single- 
celled  plant  body  and  an  antheridium  bearing  motile  male 
gametes.  The  female  gametophytes  are  richly  stored  with  food 
which  supports  the  embryo  and  young  sporophyte  until  the  latter 
becomes  adjusted  to  its  environment  and  is  self-supporting.  The 
heterosporous  selaginellas  thus  approach  the  conditions  found 
in  the  higher  seed  plants,  in  that  they  bear  two  kinds  of  spores, 
one  of  which,  the  megaspore,  is  stored  with  food  reserve  derived 
from  the  mother  plant. 


CHAPTER  XVI 

GYMNOSPERMS 
CYCADALES  (CYCADS) 

A  knowledge  of  the  life  history  of  the  cycads  will  be  of  special 
interest  and  importance  to  the  student  at  this  point,  since  botan- 
ists have  conclusively  shown  in  recent  years  that  they  are  inter- 
mediates between  the  highest  spore  plants,  represented  by  the 
heterosporous  ferns  and  selaginellas,  and  the  higher  seed  plants, 
like  the  pine  and  the  bean. 

Some  botanists  believe  that  the  flowering  plants  arose  from 
cycadlike  ancestors  in  an  earlier  geologic  period  of  the  earth's 
history,  while  all  are  agreed  that  the  modern  cycads  resemble 
closely  a- great  plexus  of  plant  groups  which  in  Carboniferous 
times  bridged  the  gap  between  the  highest  living  heterosporous 
pteridophytes  (namely,  Selaginella)  and  such  seed  plants  as  the 
pines  and  the  flowering  plants.  The  intermediate  character- 
istics of  the  cycads  have  been  shown  to  involve  not  only  the 
reproductive  features  in  their  life  history  but  also  their  general 
habit  and  vegetative  structure. 

ZAMIA 

SPOEOPHYTE 

In  Zamia  (Fig.  187)  the  leaves  resemble  those  of  ferns  in 
size  and  form,  and  the  tuberous  stem  is  covered  with  the  scale- 
like  bases  of  former  leaves,  as  in  many  tree  ferns.  The  internal 
anatomy  of  the  above  organs,  as  well  as  their  external  form,  indi- 
cates also  a  fernlike  ancestry.  The  male  and  female  strobili  are 
borne  on  separate  plants  and  resemble  in  form  and  size  the  cones 
of  pines.  The  microsporophylls  are  much  larger,  however,  than 

321 


322 


GENEBAL  BOTANY 


the  similar  structures  in  the  pine,  and  the  numerous  sporangia  are 
usually  grouped  in  sori,  like  the  ferns  (Fig.  187,  C,  a).  The  micro- 
spores  resemble  those  of  Selaginella  and  are  called  pollen  grains. 
The  megasporophylls  are  shield-shaped  and  bear  two  megaspo- 
, : .  rangia  on  each  sporo- 


FIG.  186.   The  habit  of  a  cycad,  Dioon  edule, 
growing  in  Mexico 

The  trunk  of  this  specimen  was  15  meters  (nearly 

50  feet)  high,  and  the  age  was  estimated  to  he  over 

nine  hundred  years.  After  Chamberlain 


which  exhibit  many 
characteristics  of  the 
early  types  of  ovules. 
and  seeds  found  in 
the  fossil  plants  of 
the  coal  period.  Each 
megasporangium  con- 
sists of  a  mass  of  cellu- 
lar tissue  enveloped, 
as  in  the  higher  seed 
plants,  by  an  integu- 
ment (Fig.  187,  #), 
which  forms  a  long 
micropyle  leading  to 
the  sporangial  tissue. 
A  single  megaspore  is 
ultimately  developed 
from  a  mother  cell 
which  IS  deeply  bur- 
ie(j  within  each  young 

megaSpOrailQ'ium,      as 

.      ±L   ^  5 

m  *  lg-  Io7,   />,   a. 


GAMETOPHYTES,  EMBRYO,  AND  SEED 

The  microspores  and  megaspores  both  form  gametophytes 
within  the  spores  by  germination,  as  in  Selaginella,  and  the 
female  gametophyte  and  archegonia  are  permanently  retained 
within  the  sporangium.  The  megasporangium  is  also  furnished 
with  an  integument,  which  marks  a  new  departure  in  the  plants 
studied  thus  far  in  our  consideration  of  the  great  plant  groups. 


GYMNOSPERMS 


323 


The  female  gametophyte  (Fig.  187,  B)  is  forme.d  within  the 
megaspore,  which  enlarges  as  the  gametophyte  grows,  until  it 
comes  to  occupy  most  of  the  space  within  the  megaspore  wall, 
only  a  remnant  of  the  sporangial  tissue  being  left  at  the  micro- 
pylar  end  of  the  sporangium.  At  the  micropylar  end  of  the  game- 
tophyte from  three  to  five  archegonia  are  formed.  The  male 
gametophyte  at  first  consists  of  a  single  gametophyte  cell  and  of 


Miorosporcmffia 
•Microsporangvutm 

-Microspore 

'ametophyte 
Generative  cell 
Tube  nucleus 

Gametophyte 

Stalk  cell. 
Sperm  mother 
cells 

Pollen  tube 


FIG.  187.    Habit  and  reproductive  structures  of  a  Florida  cycad,  Zamia 

A,  plant  of  Zamia  bearing  a  female  strobilus;  13,  a,  megasporophyll,  sporangia,  ard 

spores;    &,  megasporangium  (ovule)  with  gametophyte,  archegonia,  pollen  grains, 

and  pollen  tubes ;   (7,   a,  microsporophyll  and  sporangia ;  d,  e,  two  stages  in  the 

germination  of  the  microspore  and  in  the  production  of  the  male  gametophyte 


Fleshy  layer 
-^Integument 
Pollen  chamber 
Micropyle 


an  antheridial  cell,  called  the  generative  cell  (Fig.  187,  (7,  d).  The 
generative  cell  then  divides  and  there  are  ultimately  formed  a 
stalk  cell  and  two  sperm  mother  cells,  each  of  which  then  de- 
velops a  motile  sperm  (Fig.  187,  (7,  £,  /).  Pollination  takes 
place  when  the  sporophylls  of  the  female  strobili  separate  and  the 
microspores  are  borne  to  them  by  air  currents.  The  microspores 
are  drawn  into  the  micropyle  by  a  secretion  which  contracts  with 
drying  and  carries  the  spores  into  a  chamber  (called  the  pollen 
chamber)  developed  in  the  sporangial  tissue  at  the  base  of  the 
micropyle  (Fig.  187,  B).  The  pollen  tubes  developed  by  the 


324 


GENERAL  BOTANY 


microspores  bore  into  this  sporangial  tissue  and  absorb  nourish- 
ment for  the  developing  gametes.  When  the  eggs  are  ripe,  the 
sporangial  tissue  between  the  pollen  chamber  and  the  female 

gametophyte  breaks  down  and  the 
motile  male  gametes  are  cast  out  into 
a  depression  in  the  gametophyte,  called 
the  archegonial  chamber,  into  which 
the  necks  of  the  archegonia  open.  Fer- 
tilization takes  place  when  a  motile 
male  gamete  bores  through  the  neck  of 
the  archegonium  and  fertilizes  the  egg. 
Embryo  and  seed.  The  embryo  spo- 
rophyte bores  its  way  into  the  gameto- 
phyte tissue,  much  as  in  Selaginella,  by 
means  of  a  suspensor.  The  mature 
embryo  in  the  seed  consists  of  two  cotyledons,  a  hypocotyl, 
and  a  plumule  (Fig.  188).  The  seed  is  therefore  much  like 
that  of  the  higher  plants  studied  in  Part  I  and  consists  of  the 
seed  coat,  or  integument,  a  mere  remnant  of  the  megasporan- 
gium,  the  gametophyte,  and  the  embryo  sporophyte.  Zamia 


Micrvpyle 

FIG.  188.  Seed  of  Zamia  with 
embryo 


Sporophyte  plant 


Gametophyte  organs  Gametes 


Gametophyte  Sporophyte 

FIG.  189.   Life  history  of  Zamia  represented  graphically 

is  thus  the  first  plant  among  the  plant  groups  now  under  con- 
sideration which  forms  a  true  seed.  Germination  of  the  seed 
results  in  a  seedling  sporophyte  which  develops  into  the  mature 
Zamia  plant,  similar  in  habit  to  Diom  (Fig.  186). 


GYMNOSPEEMS  325 

Life  history.  It  will  be  seen  from  the  above  brief  account  of 
the  cycad  that  it  has  the  same  general  stages  in  its  life  history 
as  Selaginella  (Fig.  185).  The  new  features  are  concerned  prin- 
cipally with  the  formation  of  a  pollen  tube,  the  changed  rela- 
tions of  the  megasporangium  and  megaspore,  and  the  formation 
of  seeds.  The  retention  of  the  megaspore  in  the  sporangium, 
which  remains  longer  on  the  mother  plant  than  in  Selaginella,  is 
accompanied  in  the  cycad  by  pollination  and  the  formation  of  a 
pollen  tube  to  serve  as  an  anchoring  and  absorbing  structure 
during  the  development  of  the  motile  male  gametes.  The  per- 
manent retention  of  the  megaspore  has  also  resulted  in  the 
formation  of  a  true  seed  composed  of  the  megasporangium,  the 
garnet ophyte,  the  megaspore,  and  the  embryo  sporophyte.  When 
the  seed  germinates,  the  sporophyte  resumes  its  growth  and  gives 
rise  to  a  new  adult  cycad  plant,  or  sporophyte. 

CONIFERALES 

THE  SPRUCE  (PICEA) 
SPOROPHYTE 

Habitat  and  habit.  The  spruce  tree,  which  is  the  spore-bearing 
plant,  or  sporophyte,  has  the  same  general  form  and  mode  of 
growth  as  the  pine  tree  described  earlier  in  the  text.  Like  the 
pine  the  spruce  is  an  erect  tree  type  with  an  excurrent  trunk 
and  pyramidal  crown,  which  results  from  its  mode  of  growth  and 
the  spiral  arrangement  of  its  branches. 

The  spruce  differs  from  the  pine  in  that  its  needlelike  leaves 
are  borne  directly  and  singly  on  the  main  shoot  instead  of  in 
pairs  or  clusters  on  the  end  of  minute  dwarf  shoots.  Like  the 
pines  and  their  allies  the  spruces  also  inhabit  mainly  northern 
or  mountainous  regions  and  are  typically  xerophytic  in  habit  and 
structure,  although  they  adapt  themselves  readily  to  cultivation 
and  to  mesophytic  conditions.  In  stem  structure  the  spruces  are 
intermediate  between  the  pteridophytes  and  the  woody-stemmed 
flowering  plants,  as  the  following  account  of  the  structure  of 
the  spruce  will  indicate. 


326 


GENERAL  BOTANY 


Structure.  The  broad  outlines  of  structure  in  the  spruce  stem 
are  identical  with  those  of  the  woody  stems  of  the  alder,  ash,  and 
other  trees  and  shrubs  belonging  to  the  flowering  plants  already 

studied  (Figs.  191  and 
192).  The  bark  is  com- 
posed of  thick  layers  of 
cork,  which  scale  off  in 
flakes  on  the  main  trunk 
and  the  older  branches  of 
the  tree.  This  corky  bark, 
like  that  of  other  trees,  is 
formed  by  a  special  cork 
cambium  which  arises  in 
a  layer  of  cortical  cells 
beneath  the  epidermis  dur- 
ing the  first  year's  growth 
of  the  main  stem  or  its 
branches.  The  epidermis 
is  soon  cut  off  from  its 
water  supply  by  this  new 
growth  of  a  corky  bark, 
and  is  then  sloughed  off. 
The  cortex  forms  a  wide 
zone  of  storage  tissue  in 
the  young  stem,  but  is 
destroyed  later  by  being 
crushed  between  the  corky 
bark  and  the  expanding 
central  cylinder  of  phloem 
and  xylem.  A  distinctive 
feature  of  the  cortex  in 
the  spruces  and  pines  is  the  formation  of  the  large  resin  canals, 
which  contain  the  resin  common  in  coniferous  trees.  The  central 
cylinder  presents  the  same  general  features  as  that  of  other  trees. 
The  cambium  is  flanked  on  either  side  by  the  phloem  and  xylem 
rings,  and  the  annual  wood  rings  are  also  sharply  marked  by 
the  difference  in  structure  of  the  spring  and  summer  wood. 


FIG.  190.   Virgin  forest  of  red  spruce  (Picea 
rubra)  in  the  Adirondack  Mountains 

Photograph  furnished  by  the  United  States  Divi- 
sion of  Forestry 


GYMNOSPERMS 


327 


Cortex 
Phloem 
Xylem 


Leaf  gap 
Leaf  base 

FIG.  191.    Cross  section  of  spruce  stem 
two  years  old 


The  xylem  is  distinctive  in 
Picea  ancLthe  other  Conifer- 
ales  (including  the  common 
pines,  hemlocks,  and  cedars) 
by  being  made  up  almost 
exclusively  of  single-celled 
water-conducting  elements 
called  tracheids  (Fig.  193). 
These  single-celled  trache- 
ids resemble  those  of  the 
ferns  and  do  the  work  of 
the  long  vessels,  or  ducts, 
in  woody  and  herbaceous 
stems  of  the  flowering  plants.  The  tracheids  of  Coniferales  are 
furnished  with  peculiar  bordered  pits  representing  thin  places 
in  the  cell  wall,  which  later  become  partially  roofed  over  by 
the  extension  of  the  adjacent  thicker  portions  of  the  tracheid 
wall.  The  wood  rays 
have  also  certain  dis- 
tinctive features  in  the 
spruce  and  its  allies, 
but  their  general  struc- 
ture and  function  is  the 
same  as  that  of  the 
rays  of  the  higher  flow- 
ering plants.  The  pith 
is  small  and  is  more  or 
less  irregular  in  outline 
on  account  of  the  leaf 
gaps,  which,  as  in  the 
ferns,  cause  breaks,  or 
gaps,  in  the  otherwise 

solid  vascular  cylinder  FlG  192    Crogs  gection  of  gpmce  twig 

of  the  first  year 

The  radiate  form  of  the  pith  is  .due  to  the  numerous 
leaf  gaps  at  the  bases  of  leaf  traces,  or  vascular 
bundles,  going  out  to  the  leaves.  The  irregularities 


--Leaf  base 


of  a  young  stem. 

The  general  relation 
of  the  leaf  traces  and 
leaf  gaps  to  the  vascular 


in  the  outer  cortex  are  due  to  the  effect  of  leaf  bases 


328 


GENERAL  BOTANY 


cylinder  in  a  two-year-old  stem  of  a  spruce  are  diagrammati- 
cally  shown  in  Fig.  194.  In  the  lower  part  of  the  figure  the 
cortex  and  one  annual  ring  of  wood  are  represented  as  having 
been  removed  so  as  to  expose  the  outer  surface  of  the  wood  of 
the  first  year.  On  this  exposed  surface  the  leaf  gaps  appear  as 
in  the  vascular  cylinder  of  the  maidenhair  fern  (Fig.  170).  In 
the  upper  portion  of  Fig.  194  the  pith  is  removed  and  the  long 
leaf  gaps  are  visible  on  the  inside  of  the  wood  cylinder.  In  the 


Tracheids 


. 

Wood  ray  a  b  '"* parenchyma; 

FIG.  193.   Structure  of  the  wood  of  the  spruce  (Picea) 

a,  transverse  section ;  6,  long  section.   Copied  from  Jeffrey's  "  Anatomy  of 
Woody  Plants  " 

partial  cross-section  view  at  the  junction  of  the  upper  and  lower 
portions  of  the  figure  the  pith  is  seen  to  continue  into  the  leaf 
gaps  and  in  a  living  stem  would  be  continuous  with  the  cortex 
through  the  gap  during  the  first  year's  growth.  This  struc- 
tural feature  is  also  shown  in  the  first  annual  ring  of  wood 
as  it  appears  in  actual  sections  of  one-  and  two-year-old  stems 
of  the  spruce  (Figs.  191  and  192).  The  young  wood  cylinder  of 
the  spruce  during  the  first  year  is  therefore  similar  to  that  of  a 
fern  like  the  maidenhair  in  having  leaf  gaps  where  portions  of 
the  wood  and  phloem  cylinder  pass  out  to  form  a  leaf  trace. 
This  persistence  of  a  fern  characteristic  in  the  stem  structure  of 


GYMNOSPERMS 


329 


a  seed  plant  indicates,  according  to  the  teachings  of  modern 
anatomy,  that  the  spruces  have  been  derived  from  plants  with  a 
fern  ancestry.  In  older  spruce  stems  the  leaf  gaps  are  covered 
over  by  the  later-formed  annual  rings  of  wood,  but  they  are  still 
evident  as  radial  projections  of  the  pith.  The  leaf  traces  are, 
however,  persistent  throughout  the  life  of  the  evergreen  leaves 
and  may  often  be  Leaf  petiole 

seen  to  connect  with  rr£. -Leaf  trace 

the  leaf  gaps,  as  in 
Fig.  194.  These  leaf 
traces  serve  to  connect 
the  phloem  and  xylem 
of  the  vascular  cylin- 
der of  the  branches 
with  the  green  tissues 
of  the  needle  leaves. 
The  summary  on  the 
following  page  gives 
the  important  points 
of  similarity  between 
the  anatomy  of  the 
stem  in  the  maidenhair 
fern  and  in  the  spruce, 
and  also  the  general 
advances  in  structure 
made  by  the  Oonifer- 


bium 


Leaf  gaps 


FIG.  194.    Gross  anatomy  of  the  stem  of  a  spruce 
branch  two  years  old 

The  surface  of  the  wood  cylinder  is  exposed  in  the 
lower  half  of  the  figure  by  the  removal  of  the  cortex. 
The  inner  portion  of  the  wood  cylinder  is  shown  above 
by  the  removal  of  the  pith.  Compare  with  the  similar 


ales  as  Compared  with      fiSure  of  the  fern  rhizome  (Fig.  170) .  Note,the  breaks, 

or  leaf  gaps,  in  the  wood  cylinder,  as  in  the  fern 

the  ptendophytes. 

The  leaves  of  the  spruce  are  strictly  xerophytic  in  structure, 
as  is  shown  by  their  small  size  and  by  the  thick-walled  outer 
layers  of  cells,  which  include  both  the  epidermis  and  one  or 
more  layers  of  cells  beneath  it.  Under  this  hard  outer  cov- 
ering of  cells  the  green  mesophyll  forms  a  wide,  cortexlike 
layer  containing  chloroplastids.  The  central  cylinder  of  the 
leaf  is  occupied  at  the  base  by  two  bundles  which  join  into 
one  in  its  upper  portion.  This  vascular  system  of  the  leaf,  as 
already  indicated,  is  a  continuation  of  the  leaf  trace  connecting 


330  GENERAL  BOTANY 

the  living  mesophyll  cells  of  the  leaf  with  the  water-conducting 
tissue  of  the  main  stem  and  its  lateral  branches. 

The  root  of  the  spruce  and  its  relatives  does  not  present  any 
new  features  that  need  be  discussed  in  an  elementary  textbook. 

SUMMARY 

1.  Two  cambium  layers  are  developed  in  the  spruce,  which  enable 
it  to  increase  its  stem  in  thickness  and  to  form  a  protective  outer 
cork  jacket  which  insures  against  too  rapid  changes  in  temperature, 
loss  of  water,  and  the  attacks  of  insects  and  fungi.    In  ferns  this 
cork  jacket  is  unnecessary,  since  the  stem  is  usually  underground 
and  the  outer  skeletal  layer,  once  formed,  is  in  no  danger  of  being 
destroyed  by  the  annual  growth  of  the  stem  in  diameter. 

2.  The  growth  of  the  cambium  forms  a  wide  wood  and  phloem 
cylinder  for  conducting  the  larger  quantities  of  foods   and  water 
made  necessary  by  the  greatly  increased  leaf  exposure  of  the  spruce 
trees  and  their  allies. 

3.  Food  storage  is  provided  for  in  the  wood  rays  of  the  central 
cylinder  instead  of  in  the  pith  and  cortex,  as  in  the  ferns.    This 
provision  was  made  necessary  in  the  higher  plants  when  the  wide 
pith  and  cortex  of  the  ferns  was  gradually  eliminated  as  a  result  of 
the  secondary  production  of  wood  and  phloem  by  the  cambium. 

4.  The  leaf  traces  and  gaps  are  present  in  the  spruce,  but  they 
become  buried  by  the  secondary  products  of  the  cambium.    Their 
presence  in  the  spruce  stem  is  an  indication  of  the  relationship  of 
two  groups  of  plants  which  in  other  respects  are  widely  separated. 

Asexual  reproduction.  The  spruces  are  monoecious,  bearing 
both  staminate  and  ovulate  strobili  on  the  same  tree.  Each  stro- 
bilus  is  a  modified  shoot,  like  the  strobili  of  the  lycopods  and 
cycads,  with  a  central  axis  and  lateral  sporophylls  arranged  in  a 
spiral  form.  The  staminate  strobili  terminate  lateral  shoots  at 
the  ends  of  the  main  branches  (Fig.  195,  a),  where  they  live 
through  the  winter  in  the  bud  stage  and  first  make  their  appear- 
ance, in  temperate  climates,  early  in  May.  The  microsporophylls 
are  scalelike,  and  each  microsporophyll  bears  two  microspo- 
rangia  on  its  lower  abaxial  surface  (Fig.  197,  e,/).  A  micro- 
sporophyll with  its  two  sporangia  is  commonly  called  a  stamen, 


GYMNOSPEEMS 


331 


as  in  the  mandrake,  although  it  resembles  the  sporophylls  and 
sporangia  of  the  lycopods  and  cycads  quite  as  closely  as  it  does 
the  stamens  of  ordinary  flowering  plants.  Each  microsporangium 
produces  a  large  number  of  microspores  by  tetrad  division  of 
microspore  mother  cells,  exactly  as  in  the  ferns,  Selaginella,  and 
cycads,  so  that  the  spore-forming  processes  in  the  spruce  micro- 
sporangia  are  identical  with  those  of  the  sporangia  of  the  lower 
vascular  plants  already  studied*  The  microspores  of  the  spruce 
are  therefore  true  spores,  exactly  comparable  to  the  microspores 


FIG.  195.   Spruce  twigs  with  staminate  and  ovulate  strobili  in  May 

as  male  strobili ;  b,  female  strobili.  Note  the  erect  position  of  the  female  cones  ready 

to  receive  pollen 

of  Selaginella.  Each  microspore,  or  pollen  grain,  when  mature, 
is  furnished  with  two  expanded  sacs,  or  wings  (Fig.  198,  c), 
formed  by  the  inflation  of  the  outer  coat  of  the  microspore.  When 
the  microspores  are  ripe,  the  microsporangia  split  down  the  center 
of  each  sporangium,  or  anther  sac,  and  the  light-winged  spores 
are  widely  scattered,  thus  effecting  pollination. 

The  ovulate  strobili  grow  at  the  ends  of  last  year's  twigs, 
where  they  remain  in  the  bud  stage,  like  the  staminate  strobili, 
through  the  first  winter.  They  make  their  appearance,  in  tem- 
perate regions,  from  the  first  to  the  fifteenth  of  May,  occurring 
as  beautiful  red  erect  strobili  (Fig.  195,  5).  They  retain  this 
erect  position  for  two  or  three  weeks,  until  pollination  is  effected 


332 


GENEBAL  BOTANY 


by  the  microspores'  falling  into  the  space  between  the  open 
megasporophylls  and  the  axis  of  the  strobilus.  After  pollination 
the  megasporophylls  close  by  excessive  growth  on  the  abaxial 
surfaces,  and  the  cones  gradually  change  their  position,  owing 
to  carpotropic  movements,  finally  assuming  the  pendulous  posi- 
tion shown  in  Fig.  196.  These  ovulate  strobili  of  the  spruce  are 

more  complex  in  structure  than  the 
staminate  strobili,  since  each  ovulate 
strobilus  bears  on  its  axis  two  kinds 
of  scales,  or  modified  leaf  structures, 
instead  of  one,  as  in  the  staminate 
strobili.  The  large  scales  which  con- 
stitute the  conspicuous  part  of  the 
mature  cone,  or  strobilus,  are  the 
ovuliferous  scales,  each  of  which  bears 
two  megasporangia,  or  ovules,  at  its 
base.  These  large  ovuliferous  scales 
really  arise  as  adaxial  outgrowths 
from  very  small  scales  which  are  only 
evident  in  the  early  stages  of  the 
strobilus,  before  the  ovuliferous  scales 
have  outstripped  them  in  growth 
(Fig.  197,  c).  The  large  ovuliferous 


FIG.  196.    Spruce  cones  in 

June  after  pollination 
These  cones  were  photographed     scales  probably  represent  two  sporo- 
about  a  month  later  than  those     phylls  of  a  reproductive  branch,  which 

represented  in  Fig.  195.    Note  J      .  r  . 

the  change  in  size  and  position  of     grew  in  the  axils  of  leaves  correspond- 
the  cones  at  the  time  of  poiiina- 

tion  and  during  seed  formation 


mg  to  the  minute  scales  of  the  young 
cones.  For  our  purposes  we  may  prop- 
erly term  the  ovuliferous  scales  megasporophylls,  and  consider 
the  ovulate  strobilus  a  compound  strobilus  with  both  bracts 
and  sporophylls,  instead  of  a  simple  strobilus,  like  that  of  the 
staminate  cones. 

The  megasporangia,  or  ovules,  of  the  spruce  are  similar  to 
those  of  the  cycad,  with  a  single  integument  surrounding  the 
sporangium  tissue  proper.  Each  megasporangium  produces  a 
single  large  megaspore  (Fig.  197,  6),  which  finally  occupies 
a  considerable  portion  of  the  sporangium.  The  history  of  its 


GYMNOSPEEMS 


333 


development  shows  that  each  megaspore  is  produced  by  a  single 
mother  cell,  which  lies  deeply  buried  in  the  tissue  of  the  young 
sporangium.  This  mother  cell  divides  by  tetrad  division,  and  one 
of  the  cells  of  the  tetrad  forms  the  single  successful  megaspore. 
The  ovules  of  the  spruce,  like  those  of  the  cycad,  are  therefore 
true  megasporangia,  in  which  a  single  megaspore,  produced  by 
the  usual  processes  of  spore  formation,  is  formed  and  permanently 

tegasporophylli 

Microsporophyll 


Megasporangi 


Strobilus 


Microspores 


FIG.  197.   Megasporangia  and  microsporangia  with  spores 

a,  megasporophyll  (adaxial  view)  with  two  ovules  (megasporangia);  6,  mega- 
sporangium  in  median  section,  showing  the  single  megaspore ;  c,  portion  of  a  stro- 
bilus  in  section,  showing  megasporophylls,  bracts,  megasporangia,  and  spores; 
d,  male  strobilus ;  e,  microsporophylls  and  sporangia ;  /,  microsporophylls,  spores, 
and  sporangia  (sectional  view) 

retained  within  the  sporangium,  instead  of  being  shed,  as  in  Selag- 
inella.  As  the  megaspore  enlarges,  .it  germinates  and  produces  a 
true  cellular  gametophyte  (Fig.  198,  a).  After  fertilization  the 
megasporangium  becomes  the  seed,  furnished  with  a  hard  seed 
coat,  or  integument,  and  an  embryo  spore  plant  produced  by  the 
fertilized  egg. 

GAMETOPHYTES  AND  EMBRYO 

The  male  gametophyte  in  the  spruce  is  similar  to  that  of  the 
cycads  and  is  formed  within  the  microspore  (Fig.  198,  d)  as 
a  result  of  germination.  It  consists  at  first  of  two  cells  (the 


334 


GENERAL  BOTANY 


gametophyte  proper)  and  of  an  antheridial  cell  called  the  gener- 
ative cell.  The  generative  cell  then  divides  to  form  two  cells,  a 
stalk  cell  and  a  body  cell.  When  the  pollen  tube  forms,  the  stalk 
cell  disorganizes  and  frees  the  body  cell,  which  then  divides  in  the 
tube  to  form  two  male  cells  (Fig.  198,  e).  These  nonmotile  male 
cells  correspond  to  the  motile  sperms  of  the  cycads  and  ferns. 

The  female  gametophyte  is  formed  within  the  megaspore  by 
the  process  of  germination,  resulting  in  a- -cellular  gametophyte 
(Fig.  198,  a).  From  three  to  five  archegania  are  formed  on  this 


Tube  nucleus 

-Stalk  cell  nucleus 
\;rMale  cells 


^?— Pollen  tube-'' 

-Pollen  grain 

if — Micropyle 

Gametophyte 

FIG.  198.    Gametophytes  and  fertilization  in  the  spruce 

o,  ovule  at  the  time  of  fertilization ;  b,  archegonium  with  a  fertilized  egg  and  male 

(cf)  and  female  (9)  pronuclei ;  c,  d,  microspores  before  and  after  germination  to  form 

the  gametophyte ;  e,  pollen  tube  and  male  cells,  or  sperms 

gametophyte  at  its  micropylar  end,  each  archegonium  (6)  being 
composed  of  a  large  egg  cell,  a  layer  of  cells  called  the  jacket 
cells,  and  the  neck  cells.  Pollination  is  effected  by  means  of 
the  wind  when  the  young  female  cones  are  erect  on  the  ends 
of  the  branches.  The  cone  scales  are  then^open  (Fig.  195,  6),  and 
the  pollen  sifts  down  "between  them  and  comes  to  rest  in  con- 
tact with  the  micropyles  of  the  ovules.  A  sticky  secretion  is  * 
exuded  by  the  micropyle,  as  in  the  cycads,  which  draws  the 
microspore  into  the  micropyle  until  it  rests  on  the  surface  of 
the  megasporangium.  No  distinct  pollen  chamber  is  formed  in 
the  spruce,  like  that  in  the-cycad  megasporangium.  The  pollen 
tube  begins  to  grow  down  into  the  megasporangium  early  in 
May,  soon  after  pollination,  and  reaches  the  archegonia  late 


GYMNOSPERMS 


335 


in  June  (Fig.  198,  a).  Just  before  fertilization  the  end  of  the 
pollen  tube  penetrates  the  neck  of  an  archegonium  and  then 
ruptures,  liberating  the  male  cells  in  contact  with  the  egg. 
The  union  of  one  of  these  male  cells  with  the  egg  cell  com- 
pletes ttye  process  of  fertilization  and  initiates  the  formation  of 
the  embryo. 

Embryo.    As  soon  as  fertilization  has  taken  place,  the  con- 
jugate nucleus,  formed  by  the  union  of  the  male  and  female 


i 


Cotyledons 


—Hypocotyl 


a  Mlcropyk          Micropyle^ 

FIG;  199.   The  ovule,  seed,  and  seedling  of  the  spruce 

a,  ovule  at  the  time  of  fertilization';  b,  two  embryos  developing  as  a  result  of  fertili- 
zation ;  c,  seed  developed  from  a  with  only  one'embryo ;  d,  young  seedling  developed 
from  a  seed  by  germination  and  growth 

pronuclei,  divides  to  form  eight  nuclei,  which  then  pass  to  the 
bottom  of  the  egg.  Around  these  nuclei  eight  cells  are  ulti- 
mately formed,  which  constitute  the  beginning  of  the  pro- 
embryo.  This  proembryx)  soon  differentiates  into  a  suspensor, 
composed  of  four  greatly  elongated  cells,  and  the  embryonic 
cells  which  are  to  form  the  embryo  proper  (Fig.  199,  J).  The 
embryonic  cells  finally  produce  the  embryo  within  the  seed. 
This  embryo  is  composed  of  the  'hypocotyl,  or  stem,  the  root, 
and  numerous  first  leaves,  or  cotyledons,  surrounding  the  ter- 
minal plumule,  or  bud.  These  structures  are  shown  more 
plainly  in  Fig.  199,  6?,  which  represents  a  seedling  sporophyte  of 
the  spruce  produced  by  the  germination  of  the  seed.  The  seed 


336  GENEBAL  BOTANY 

is  thus  composed  of  the  seed  coat,  or  integument  (which  forms 
a  part  of  the  mother  sporophyte  plant),  of  the  gametophyte,  and 
of  the  young  sporophyte,  or  embryo,  which  represents  a  new 
sporophyte  generation  (Fig.  199,  c). 

Life  history.  The  life  history  of  the  spruce  is  similar  in  all 
essential  respects  to  that  of  the  cycads,  represented  by  Zamia. 
In  both  instances  the  megaspore  is  permanently  retained  in  the 
megasporangium.  In  the  spruce  and  its  relatives  the  male 
gametes  have  lost  their  motile  organs,  and  the  pollen  tube  is 
consequently  used  to  convey  them  to  the  eggs.  Correlated  with 
this  change  we  find  that  the  spruce  has  no  archegonial  chamber, 
since  the  pollen  tubes  enter  the  archegonial  necks  and  intro- 
duce the  male  gametes  directly  to  the  eggs.  The  results  of  ferti- 
lization are  the  development  of  the  embryo  and  the  formation 
of  a  seed.  In  order  to  have  these  points  of  difference  between 
the  cycads  and  the  spruce  clearly  in  mind  the  student  should 
construct  a  graphical  history  of  the  spruce  similar  to  that  of 
Zamia  (Fig.  18.9> 


CHAPTER  XVII 

ANGIOSPERMS 

DICOTYLEDONS 

SPOROPHYTES 

The  sporophytes  of  the  angiosperms  include  the  common 
herbaceous  and  woody-stemmed  plants,  such  as  the  mandrake, 
clovers,  and  elms,  with  which  we  became  familiar  in  the  first  part 
of  the  text.  On  account  of  the  large  amount  of  time  already 
devoted  to  the  vegetative  and  reproductive  structures  of  this 
important  group  of  plants  it  will  only  be  necessary  at  this  point 
to  review  the  knowledge  already  gained  and  to  relate  the  life 
history  of  angiosperms  to  the  higher  spore  and  seed  plants  which 
we  have  recently  considered.  In  this  discussion  the  angiosperms 
with  two  cotyledons  in  the  embryo,  namely,  the  dicotyledons, 
have  been  chosen  to  represent  the  group,  while  the  monocoty- 
ledons will  be  reserved  for  a  separate  and  special  treatment. 

Structure.  In  connection  with  the  following  brief  summary 
of  the  important  advances  in  anatomy  made  by  the  dicotyle- 
dons the  student  should  consult  the  figures  and  review  the 
text  relating  to  the  structure  of  woody  and  herbaceous  stems 
in  Part  I,  and  also  the  structure  of  Adiantum  and  the  spruce 
in  Part  II. 

The  advances  in  structure  relate  mainly  to  the  stem  tissues, 
since  the  leaves  of  dicotyledons  are  not  much  more  highly 
organized  than  those  of  ferns  and  cycads. 

The  general  arrangement  of  the  stem  tissues  in  the  woody 
types  of  dicotyledons  is  very  similar  to  that  of  the  spruce,  and 
the  advances  made  by  the  spruce  in  this  respect,  as  compared 
with  the  pteridophytes,  apply  to  the  trees  and  shrubs  among 
dicotyledons. 

337 


338  GENEBAL  BOTANY 

The  herbaceous  dicotyledons  resemble  the  herbaceous  ferns 
in  having  a  wide  storing  cortex,  a  large  pith,  and  a  narrow  vas- 
cular cylinder.  They  differ  from  the  ferns  and  are  like  the  woody 
dicotyledons  in  the  collateral  structure  of  the  phloem  and  xylem 
and  in  the  nature  of  the  tissue  elements,  which  are  essentially 
the  same  in  all  dicotyledonous  stems. 

The  storage  system  of  cells  in  the  dicotyledons,  particularly 
in  trees  and  shrubs,  is  for  the  first  time  amply  provided  for  by 
large  and  small  wood  rays  and  by  wood  parenchyma  abundantly 
distributed  throughout  the  primary  and  secondary  wood.  In 
living  pteridophytes  the  wood  rays  are  lacking,  and  gymnosperms 
have  neither  the  rays  nor  the  wood  parenchyma  so  largely  de- 
veloped as  in  the  woody  dicotyledons.  (Compare  the  figures 
illustrating  the  anatomy  of  Adiantum,  alder,  and  spruce.)  This 
highly  developed  storage  system  of  the  woody  dicotyledons  com- 
pensates for  the  small  size  of  the  pith  and  cortex,  which  serve 
the  storage  function  for  a  short  time  only  in  these  plants,  since 
the  death  of  the  cortex  and  pith  in  old  dicotyledonous  stems 
relegates  the  entire  storage  function  to  the  wood  rays  and  the 
parenchyma  of  the  vascular  cylinder. 

The  conducting  cells  are  the  familiar  ducts  which  constitute 
long  tubes  for  the  rapid  transfer  of  water  necessitated  by  the 
immense  leafage  of  the  broad-leaved  dicotyledons. 

The  average  length  of  the  tracheids  which  compose  the  water- 
conducting  elements  of  gymnosperms  is  from  two  to  four  milli- 
meters, while  that  of  the  ducts  in  dicotyledons  ranges  from  a 
few  centimeters  to  several  feet  in  length.  The  ducts,  therefore, 
offer  much  less  resistance  to  the  rapid  flow  of  water  up  the  tree 
trunk  in  a  dicotyledon  than  the  tracheids  do  in  a  spruce  or  other 
gymnosperm  (compare  Figs.  53  and  193). 

The  great  differentiation  in  kind  and  arrangement  of  tissues 
in  the  stems  of  dicotyledons  is  also  a  distinctive  feature  in  these 
plants,  since  ducts  of  various  kinds  and  sizes,  strengthening  fibers 
and  tracheids,  wood  rays  and  storage  parenchyma,  are  all  adapted, 
in  them  as  in  no  other  plants,  to  the  proper  performance  of  their 
respective  functions.  This  elaborate  differentiation  of  tissues 
culminates  in  the  woody-stemmed  trees  and  shrubs. 


AKGIOSPERMS 


339 


The  flower.  The  flower  in  the  angiosperms  presents  some  new 
and  distinctive  features  which  are  common  to  both  dicotyledons 
and  monocotyledons.  The  important  advances  made  by  the 
flower,  as  compared  with  the  strobilus  of  plants  below  the  angio- 
sperms, relate  to  the  development  of  a  showy  perianth  and  of  a 
closed  megasporophyll,  or  pistil,  and  to  certain  modifications  in 
the  relation  and  number  of  sporophylls  borne  on  the  floral  axis, 
or  receptacle.  These  new  features  can  be  most  easily  presented 


FIG.  200.    Diagram  designed  to  illustrate  the  corresponding  parts  of  the  spruce 
strobili  and  the  flower  of  the  marsh  marigold  (Caltha  palustris) 

a,  flower  of  the  marigold;   6,  section  of  the  flower;    c,  median  long  section  of  the 
staminate  strobilus  of  the  spruce ;  d,  similar  section  of  the  ovulate  strobilus 

by  instituting  a  comparison  between  a  simple  flower  like  that 
of  the  marsh  marigold  (Caliha  palustris)  and  the  strobili  of 
a  gymnosperm  like  the  spruce  (Fig.  200).  In  the  marigold 
flower  the  perianth  and  the  numerous  stamens  and  pistils  are 
arranged  in  a  spiral  form  on  a  dome-shaped  receptacle  like  the 
sporophylls  on  the  axis  of  a  spruce  cone.  Such  flowers  with 
spirally  arranged  parts  evidently  correspond  more  nearly  to  the 
strobilus  of  the  plants  below  them  than  the  cyclic  flowers  of  the 
mandrake  and  locust,  in  which  the  separate  sets  oL  floral  organs 
are  arranged  in  cycles  on  a  flattened  receptacle.  If  a  median 
longitudinal  section  of  a  marigold  flower  is  compared  with  sim- 
ilar sections  of  the  male  and  female  strobili  of  the  spruce,  the 


340  GENERAL  BOTANY 

corresponding  parts  of  the  flower  of  the  angiosperms  and  the 
strobili  of  the  gymnosperms  are  at  once  made  apparent. 

The  receptacle  of  the  flower  evidently  corresponds  to  the  axis 
of  a  strobilus,  although  it  is  greatly  shortened  and  somewhat 
flattened  at  its  apex.  The  stamens  correspond  to  the  microsporo- 
phylls  and  microsporangia  of  the  spruce  strobilus,  the  filament 
representing  a  highly  modified,  slender  microsporophyll,  and  the 
anther  sacs  representing  microsporangia  borne  at  the  apex  of  the 
sporophyll.  A  single  pistil  of  the  marsh  marigold  flower  corre- 
sponds to  a  single  megasporophyll  on  the  female  strobilus  of  the 
spruce,  in  which  the  edges  have  folded  in  and  united  so  as  to 
inclose  the  megasporangia,  or  ovules,  in  a  cavity  called  in  the 
angiosperm  flower  the  ovary  cavity. 

The  perianth  is  evidently  a  new  structure  which  functions  to 
protect  the  essential  organs,  the  stamens  and  pistils,  during  their 
development.  We  have  learned  that  in  highly  organized  flowers 
like  the  locust  and  the  bean  the  perianth  may  also  serve  an 
important  function  in  securing  cross-pollination  by  insects.  The 
perianth  in  some  flowers  undoubtedly  represents  transformed 
sporophylls  at  the  base  of  a  strobiluslike  flower,  while  in  other 
instances  it  is  apparently  formed  from  ordinary  green  leaves 
below  the  sporophylls.  We  may  conclude,  therefore,  that  the 
angiosperm  flower,  represented  by  the  flower  of  the  marsh  mari- 
gold, is  a  highly  modified  strobilus,  in  which  many  changes  have 
taken  place  during  its  long  course  of  evolution,  including  the 
shortening  of  the  axis,  or  receptacle,  and  the  transformation  of 
simple  sporophylls  and  sporangia  into  stamens  and  pistils  and 
of  certain  sporophylls,  or  green  leaves,  into  the  parts  of  the 
perianth,  namely,  the  calyx  and  corolla. 

The  evolution  of  the  sporophylls  and  sporangia  of  the  angio- 
sperm flower  will  be  more  fully  understood  if  a  further  com- 
parison is  made  between  these  structures  and  the  corresponding 
structures  in  the  other  vascular  plants  already  studied.  In 
Fig.  201  the  microsporophylls  and  sporangia  of  Selaginella^  spruce, 
and  marsh  marigold  are  compared  with  a  portion  of  the  sporo- 
phyll, or  leaf,  of  Adiantum.  From  this  figure  it  will  be  seen 
that  the  sporophyll  has  become  gradually  reduced  in  size  from 


ANGIOSPEKMS 


341 


the  fern  to  the  angiosperm,  until  it  has  reached  its  limit  in 
the  slender  filament  of  the  angiosperm  stamen.  With  this  grad- 
ual reduction  in  size  its  original  chlorophyll  tissue  has  been  lost, 
together  with  its  power  of  making  starch,  so  that  the  microsporo- 
phyll  now  serves  a  single  function,  namely,  that  of  producing 


Microsporophylls     Microsporophyl 


Micro- 
sporangici 


Micro  - 
sporangia 


Megasporophylls 


_Mega- 
'/sporancjid 


Fig.  201.    Diagram  illustrating  the  homologous,  or  corresponding,  parts  of  the 
sporophylls  and  sporangia  of  the  higher  spore  and  seed  plants 

«,  pinnule  (sporophyll)  and  sporangium  of  the  maidenhair  fern ;  6,  microsporophyll 
and  microsporangium  of  Selaginella ;  c,  d,  corresponding  parts  of  the  anthers  of 
the  spruce  and  of  the  marsh  marigold ;  e,  pinnule  (sporophyll)  and  sporangium  of  the 
maidenhair  fern;  /,  megasporophyll  and  megasporangium  of  Selaginella;  g,  ovu- 
lif erous  scale  and  ovules  of  the  spruce ;  h ,  ovary  and  ovules  of  the  marsh  marigold 

microspores,  instead  of  the  double  function  of  spore  production 
and  photosynthesis,  as  in  the  ferns.  The  sporangia  have  also 
become  transformed  from  the  simple,  distinct  sporangia  of  the 
ferns  .into  the  four  united  microsporangia  of  the  angiosperms, 
borne  .on  a  single  sporophyll. 

The  pistil,  which  may  be  composed  of  a  single  megasporophyll,  as 
in  the,  marsh  marigold,  or  of  several  megasporophylls,  as  in  the  lily 
or  the  apple,  represents  a  still  greater  transformation  in  sporophyll 


342  GENERAL  BOTANY 

structure  than  that  outlined  above  for  the  microsporophyll.  A 
single  pistil  of  the  marigold  evidently  corresponds  to  one  mega- 
sporophyll  of  the  spruce,  with  its  edges  turned  in  and  united  to 
form  the  ovary  cavity.  At  the  point  of  union  of  the  edges  of  the 
sporophyll,  which  form  the  placenta,  the  megasporangia,  or  ovules, 
bud  out  and  develop  the  integuments  and  sporangium  proper, 
which  are  characteristic  of  the  megasporangia  of  the  seed  plants. 
At  the  apex  of  the  megasporophyll  the  stigma  is  developed,  which 
in  the  marigold,  as  in  many  other  flowers,  is  furnished  with  hair- 
like  outgrowths  for  the  retention  of  pollen.  This  highly  modified 
megasporophyll,  or  pistil,  is  the  most  universally  characteristic 
and  important  feature  of  the  angiosperm  flower.  Such  a  pistil 
as  that  of  the  marigold,  which  is  composed  of  a  single  mega- 
sporophyll, is  called  a  simple  pistil,  to  distinguish  it  from  the  com- 
pound pistils  like  that  of  the  tulip  or  the  apple,  in  which  more 
than  one  megasporophyll  enters  into  the  composition  of  the 
ovary.  The  compound  pistil  is  therefore  a  union  of  several 
simple  pistils  into  one  structure. 

It  is  an  interesting  fact  that  the  development  of  a  pistil  in 
the  young  flower  of  a  marigold  or  a  buttercup  corroborates  the 
above  interpretation  of  the  probable  origin  of  the  closed  angio- 
sperm pistil  from  an  open  megasporophyll  similar  to  that  of 
gymnosperms.  The  young  sporophyll  which  buds  out  on  the 
receptacle  of  a  developing  flower  of  a  marigold  or  a  buttercup  is 
at  first  an  open  sporophyll  resembling  a  rudimentary  spruce 
sporophyll  but  with  a  concavity  toward  the  axis  of  the  flower. 
The  edges  of  this  concave  sporophyll  gradually  approach  each 
other  by  growth  and  finally  unite  to  form  the  ovary  cavity. 
Meanwhile  the  megasporangia,  or  ovules,  bud  out  upon  the 
uniting  edges  where  the  placenta  is  to  be  formed,  and  the  stigma 
develops  at  the  apex  of  the  leaflike  sporophyll.  By  further 
growth  the  mature  closed  pistil  of  the  marigold  is  formed. 

Asexual  reproduction.  The  anther  of  the  angiosperms  is  usu- 
ally composed  of  four  lobes,  visible  from  the  outside,  which 
represent  four  microsporangia.  These  four  microsporangia  are 
shown  in  Fig.  202,  A,  as  they  appear  in  a  transverse  section 
of  a  mature  anther  of  a  lily.  In  a  younger  anther  than  that 


ANGIOSPEEMS 


343 


represented  in  the  figure  the  pollen  grains  would  be  replaced 
by  mother  cells,  which  form  the  pollen,  or  microspores,  by  tetrad 
division,  as  in  ferns,  cycads,  and  spruces.  The  mother  cell  first 


FIG.  202.   Anther  and  pollen  formation  in  a  lily 

A,  mature  anther  with  four  microsporangia  containing  pollen  grains ;  B,  the  process 

of  forming  microspores  by  tetrad  division ;   (7,  gametophyte  (g)  and  tube  nucleus  (t) 

in  a  germinated  microspore.   From  Bergen  and  Davis 's  "  Principles  of  Botany  " 

divides  into  two  cells  (5),  with  a  reduction  in  the  number  of 
chromosomes  (consult  Fig.  43,  5).  These  two  cells  then  divide 
again  and  form  the  four  cells  of  the  tetrad.  Each  cell  of  the 


344  GENERAL  BOTANY 

tetrad  then  develops  into  a  microspore,  or  pollen  grain.  When 
the  pollen  grains  are  ready  to  be  shed,  the  cellular  partition 
separating  the  two  microsporangia  on  each  side  of  the  anther 
breaks  down,  and  the  two  anther  sacs  are  thus  formed.  The 
anther  wall  then  ruptures  along  the  line  of  dehiscence  and  sheds 
the  microspores  (consult  Fig.  202,  A,  x). 

The  young  megaxporangia,  or  ovules,  arise  in  the  shepherd's 
purse  (Capsella)  from  two  placentae,  formed  at  the  junction  of 
the  two  sporophylls  in  the  ovary.  Each  megasporangium,  when 
young  (Fig.  203,  A),  consists  of  a  sporangium  proper,  called  the 
nucellus,  and  of  two  cellular  outgrowths  at  the  base  of  the 
sporangium,  which  are  the  beginnings  of  the  outer  integument 
and  the  inner  integument.  The  funiculus  is  not  perfectly 
developed  in  the  young  ovule,  but  as  the  megasporangium 
increases  in  size  it  grows  more  rapidly  on  one  side  than  on  the 
other,  which  gives  it  a  curved  form  (Fig.  203,  j5).  Coincident 
with  these  changes  in  form  a  single  cell  of  the  sporangium 
enlarges  and  becomes  the  mother  cell  of  the  future  megaspore. 
In  Capsella  this  cell  divides  into  a  row  of  three  cells  which  are 
potential  megaspores,  and  the  lowest  of  the  three  then  enlarges 
ind  forms  a  large  megaspore,  such  as  is  shown  in  B,  es.  In  many 
angiosperms  four  cells  arise  from  the  megaspore  mother  cell 
instead  of  three,  as  in  Capsella,  which  indicates  that  these  cells 
constitute  a  spore  tetrad.  This  process  relates  the  formation 
of  the  megaspore  in  the  angiosperms  to  the  usual  process  of 
sporogenesis  as  it  occurs  in  Selaginella  and  in  the  microspores 
of  angiosperms.  The  megaspore,  when  it  has  reached  the  size 
shown  in  Fig.  203,  B,  germinates  at  once  and  forms  a  female 
gametophyte  (<7)  exactly  like  that  already  described  in  the 
mandrake.  This  germinated  megaspore  is  called  the  embryo  sac. 


GAMETOPHYTES  AND  FERTILIZATION 

The  female  gametophyte  of  Capsella  (Fig.  203,  C)  corresponds 
exactly  to  that  of  the  mandrake  (Fig.  86,  5)  and  the  iris 
(Fig.  204).  It  consists  of  the  egg  cell,  or  female  gamete,  the 
synergidae  (which  are  closely  associated  with  the  egg),  the  polar 


ANGIOSPEBMS 
c 


345 


FIG.  203.   Development  of  the  ovule  and  embryo  of  shepherd's  purse  (Capsella) 

A-C,  stages  in  the  development  of  the  ovule  ;  n,  nucellus  of  megasporangium  ;  ii  and 

ol,  inner  and  outer  integuments ;    m,  micropyle ;   es,  embryo  sac ;    //,  mature  ovule 

(megasporangium)  with  emhryo  sac,  embryo  (em),  and  endosperm  nuclei  (e);  D~G, 

stages  in  the  development  of  the  embryo ;  s,  suspensor ;  r,  root ;  c,  cotyledons 

nuclei,  and  the  antipodal  cells.  The  student  will  note  at  once 
the  great  difference  between  this  reduced  gametophyte  of  the  an- 
giosperms  and  that  of  the  cycads  and  spruce.  In  the  angiosperm 
the  female  gametophyte  consists  of  six  cells  and  two  nuclei, 


346  GENERAL  BOTANY 

while  in  the  gymnosperms  (cycad,  spruce,  and  pine)  the  gameto- 
phyte  is  a  definite  cellular  structure  which  bears  true  archegonia. 
The  male  gametopliyte  is  also  greatly  reduced  in  Capsella,  as 
in  the  mandrake  (Fig.  85,  6)  and  the  iris  (Fig.  204,  a).  Pollina- 
tion and  fertilization  take  place  in  the  manner  already  described 
for  the  mandrake  and  the  bean.  Double  fertilization  probably 
occurs  in  Capsella,  although  this  has  not  been  definitely  investi- 
gated. In  this  process,  as  shown  in  Fig.  204,  6,  the  nucleus  of 
one  of  the  male  cells  unites  with  the  egg,  while  that  of  the  other 
combines  with  the  polar  nuclei  to  form  the  endosperm  nucleus. 
The  fertilized  egg  cell  develops  into  the  embryo,  while  the  endo- 
sperm nucleus  initiates  the  formation  of  the  endosperm. 


THE  EMBRYO,  ENDOSPERM,  AND  SEED 

The  embryo.  After  fertilization  the  egg  secretes  a  cellulose 
wall  and  then  divides  by  repeated  mitoses,  forming  an  elon- 
gated, rodlike  proembryo  (Fig.  203,  D).  The  upper  cell  of  this 
proembryo  forms  the  embryo  proper  by  cell  division  and  differ- 
entiation, while  the  remaining  cells  form  the  suspensor  (E,  F) 
which  supplies  the  embryo  with  nutriment  and  anchors  it  within 
the  megaspore,  or  embryo  sac.  The  two  cotyledons  bud  out 
from  the  upper  half  of  the  embryo,  while  the  root  grows  from 
the  lower  portion  (6r).  In  If  the  proembryo  is  shown  in  posi- 
tion in  the  embryo  sac,  with  the  lower  cell  of  the  suspensor 
occupying  an  enlarged  portion  of  the  sac  next  to  the  micropyle. 

The  endosperm.  In  Capsella,  as  in  other  angiosperms,  the 
endosperm  begins  to  form  by  repeated  divisions  of  the  endosperm 
nucleus,  formed,  as  indicated  above,  by  the  union  of  one  male 
nucleus  and  the  two  polar  nuclei. 

The  numerous  endosperm  nuclei  thus  formed  accumulate  in 
the  peripheral  layer  of  cytoplasm  which  surrounds  the  large 
central  vacuole  of  the  embryo  sac,  where  they  form  a  layer  of 
free  nuclei  (Fig.  203,  H,  e).  These  free  nuclei  never  form  a  per- 
manent cellular  endosperm  in  Capsella,  since  they  are  gradually 
absorbed  by  the  growing  embryo,  which  in  a  later  stage  of  its 
development  fills  the  embryo  sac. 


ANGIOSPERMS 


347 


The  seed.  In  the  ripened  seed  of  Capsella  the  embryo  fills  the 
entire  cavity  of  the  embryo  sac ;  the  food  reserve  necessary  for 
its  growth  during  seed  germination  is  stored  in  the  cotyledons, 
as  in  the  pea  and  bean.  The  embryo,  as  in  the  latter  seeds,  has 
two  cotyledons,  a  plumule  (or  first  terminal  bud),  a  hypocotyl, 
and  the  root  meristem  at  the  tip  of  the  hypocotyl.  The  seed 
coats  are  formed  of  cells  with  greatly  thickened  walls,  which 


ale  cell — fc- 
,( sperm) 

Tube  nucleus 
—Pollen  tube''' 


FIG.  204.    Ovule,  pollen  tube,  and  fertilization  in  Iris  and  a  lily 

a,  ovule  of  Iris  with  embryo  sac  and  female  gametopliyte  at  the  time  of  fertiliza- 
tion ;  6,  double  fertilization  in  a  lily ;   c,  pollen  tube  of  Iris,    b,  after  Gu'inguard ; 
a  and  c,  from  original  drawings  by  M.  Louise  Sawyer 

effectually  protect  the  embryo  during  its  period  of  rest.  The 
germination  of  the  seed  and  the  adjustments  of  the  seedling  to 
the  environment  are  essentially  the  same  as  in  the  bean. 

Life  history.  It  is  evident  from  the  above  discussion  that  the 
angiosperms  have  the  same  general  stages  in  their  life  history  as 
the  spruce.  The  new  features  relate  to  details  of  structure  and 
reproduction  already  discussed  and  hence  need  only  a  brief 
treatment  in  the  form  of  a  summary  at  this  point. 

The  flower  of  the  angiosperms  is  a  Highly  modified  strobilus  in 
which  the  microsporophylls  have  been  transformed  into  stamens 
and  the  megasporophylls  into  one  or  many  pistils.  The  closed 
pistil,  with  the  stigma  differentiated  for  the  reception  of  pollen, 
is  the  most  distinctive  feature  of  the  angiosperms,  although  the 
perianth,  when  developed,  is  an  important  characteristic.  The 


348 


GENERAL  BOTANY 


perianth  is  usually  composed  of  modified  sporophylls,  but  in 
some  flowers  the  leaves  immediately  below  the  sporophylls  have 
been  transformed  into  sepals  and  petals. 

Pollination  devices  are  more  highly  developed  in  the  angio- 
sperms  than  in  any  of  the  gymnosperms,  and  the  pollen  tube 
traverses  the  tissues  of  the  style,  stigma,  and  ovary  cavity  before 
coming  in  contact  with  the  micropyle.  The  development  of  a 
closed  pistil  with  a  receptive  stigmatic  surface  has  therefore  greatly 
modified  both  pollination  and  the  growth  of  the  pollen  tube. 

The  gametophytes  are  greatly  reduced  in  size  and  in  cellular 
differentiation.  The  male  gametophyte  is  represented  by  the 
single  generative  cell,  which  gives  rise  to  the  two  male  cells 


Selaginella 


Spruce 


Mandrake 


Nuclei 
r*~  Cytoplasm— 


'I  Gametophyte,^ 


FIG.  205.   Diagram  illustrating  the  homologous,  or  corresponding,  structures  of 
microspores  and  male  gametophytes  in  Selaginella,  spruce,  and  mandrake 

a-c,  microspores ;  d-f,  germinated  microspores  and  gametophytes 

within  the  germinated  pollen  grain,  or  microspore.  The  female 
gametophyte  is  reduced  to  the  egg  apparatus,  polar  nuclei,  and 
antipodals  in  the  embryo  sac.  The  embryo  develops  from  the 
fertilized  egg  cell  within  the  embryo  sac  and  passes  into  a  rest- 
ing stage  within  the  seed.  The  full  development  of  the  sporo- 
phyte  begins  with  seed  germination,  when  the  embryo  resumes 
its  growth,  fed  by  the  endosperm  stored  up  around  the  embryo 
or  within  its  cotyledons. 

SUMMARY  AND  COMPARISONS 

A  brief  summary  and  comparison  of  the  heterosporous  plants 
from  Selaginella  to  the  angiosperm  will  suffice  to  indicate  the  strik- 
ing advances  made  by  the  highest  spore-bearing  and  seed  plants  in 


ANGIOSPEEMS 


349 


the  evolution  of  the  seed  and  the  seed  habit.  The  more  important 
of  these  changes  are  represented  graphically  in  Figs.  205  and  206. 
In  these  figures  it  may  be  seen  that  the  microspores  have  remained 
quite  similar  in  form,  size,  and  structure,  since  their  function  has 
not  changed.  In  the  angiosperm,  as  in  Selaginella,  the  microspores 
still  serve  the  function  of  bearing  the  male  gametes  to  the  female 
gametes,  and  are  hence  small,  light,  and  highly  protected  cells,  pro- 
duced in  great  numbers  by  the  microsporangia  to  guard  against 
waste  in  distribution 

oelagtnelut  spruce 

by  wind  or  insects. 

The  male  gameto- 
phyte  has  been  grad- 
ually reduced,  as  was 
explained  in  a  previous 
paragraph,  until  it  is 
represented  in  the  an- 
giosperm by  a  single 
generative  cell.  The 
megaspores  (Fig.  206) 
have  become  reduced 
to  a  single  megaspore 
in  each  megasporan- 
gium,  both  in  the  gym- 
nosperms,  represented 
by  the  spruce,  and  in 
the  angiosperms,  rep-  FIG.  206.  Diagram  showing  the  homologous,  or 
resented  by  the  man-  corresponding,  parts  of  megaspores,  megasporan- 

g^metophytes,  and  sporophytes  of  Selaginella, 
spruce,  and  mandrake 


a-c,  megaspores;    d-f,   gametophytes  ;    g-i,  gameto- 
phytes  and  sporophytes.   A  and  i  are  seeds 


drake.     The  important 

advance  made  by  the 
-,    ,  P-, 

latter  groups  of  plants 

over  Selaginella  and 
similar  heterosporous  Pteridophyta  is  that  of  retaining  the  mega- 
spore  permanently  in  the  sporangium.  The  megatporanpium  thus 
becomes  indehiscent  and  is  shed  from  the  mother  plant  and  dis- 
seminated with  the  contained  megaspore  and  embryo.  The  female 
gametophytes  remain  cellular  structures  produced  by  the  germina- 
tion of  the  megaspore  in  both  Selaginella  and  the  spruce.  In  Selagi- 
nella, however,  the  gametophyte  and  archegonia  are  still  exposed 
by  the  opening  of  the  megasporangium  and  spore.  In  the  indehis- 
cent megasporangium  of  the  spruce  the  cellular  gametophyte  is 
retained  permanentlv  in  the  spore  within  the  megasporangium,  but 


350  GENERAL  BOTANY 

it  still  bears  definite  archegonia  with  neck  cells  and  a  ventral-canal 
cell  nucleus  inherited  from  its  fernlike  ancestors  of  the  coal  period. 

In  the  angiosperm  no  cellular  gametophyte  is  formed  before  fer- 
tilization, and  the  gametophyte  is  reduced  to  the  egg  apparatus, 
polar  nuclei,  and  antipodals.  Pollination  and  fertilization  methods 
are  also  greatly  modified,  in  both  the  higher  gymnosperms  and 
angiosperms,  by  the  permanent  inclosure  of  the  gametophyte  in  the 
megasporangium.  In  Selaginella  the  motile  sperms  reach  the  eggs 
in  the  exposed  archegonia  by  swimming  in  films  of  water.  In  cycads 
the  sperms  are  still  motile,  with  cilia,  but  they  are  liberated  in  a 
special  chamber,  called  the  archegonial  chamber,  into  which  the 
mouths  of  the  archegonia  open.  In  the  higher  gymnosperms  and 
angiosperms,  however,  fertilization  depends  upon  the  growth  of  the 
pollen  tube  through  the  micropyle  down  to  the  egg ;  by  this  growth 
a  canal  is  formed,  down  which  the  nonmotile  male  gametes  reach 
and  fertilize  the  eggs.  This  is  true  siphonogamy,  or  the  fertilization 
of  the  egg  through  the  intermediary  of  a  pollen  tube.  In  the  angio- 
sperms the  development  of  a  pistil,  with  stigma  and  style,  renders 
pollination  and  fertilization  even  more  difficult  and  is  correlated 
with  the  elaborate  devices  for  securing  pollination  observed  in  many 
angiosperm  flowers. 

The  seed  is  a  complicated  structure  composed  of  the  megasporan- 
gium of  the  mother  plant  and  the  embryo.  In  the  angiosperms  the 
true  gametophyte  of  the  gymnosperms  is  replaced  in  many  species 
by  the  endosperm,  which,  as  we  have  learned,  is  a  nutritive  tissue 
formed  as  a  result  of  fertilization.  In  'other  respects  the  seeds  of 
angiosperms  and  gymnosperms  are  quite  similar  in  structure.  The 
most  striking  advances  and  changes  leading  to  the  evolution  of 
seeds  are,  therefore,  the  reduction  of  both  male^and  female  gameto- 
phytes,  the  reduction  in  the  number  of  megaspores  produced  by  the 
megasporangia,  the  retention  of  the  megaspores  and  female  gameto- 
phy  tes  permanently  within  the  sporangia,  and  the  changes  in  methods 
of  pollination  and  fertilization  correlated  with  this  retention. 


PART   III.  REPRESENTATIVE  FAMILIES  AND 
SPECIES  OF  THE  SPRING  FLORA 


CHAPTER  XVIII 

DESCRIPTIVE  TERMS 

For  the  study  of  plants  in  the  field  the  student  will  need  cer- 
tain descriptive  terms  which  have  not  been  given  in  the  preceding 
pages.  In  the  following  brief  discussion,  therefore,  we  shall  define 
the  more  important  descriptive  terms  which  the  student  will 
need  in  studying  the  trees,  shrubs,  and  herbaceous  plants  of  the 
spring  flora. 

VEGETATIVE  PARTS  OF  PLANTS 

Habitat.  The  term  habitat  is  used  to  indicate  the  nature  of  the 
environment  in  which  individual  plants  or  plant  groups  live. 
The  most  common  classification  of  habitats  is  that  already 
described,  based  upon  the  conditions  of  moisture,  soil,  and  light 
which  constitute  the  environment.  Habitats  may  therefore  be 
designated  as  metophytie,  xerophytic,  hydrophytic,  and  tropophytic, 
according  as  the  plants  inhabiting  these  areas  are  mesophytes, 
xerophytes,  hydrophytes,  or  tropophytes  in  habit. 

Habit.  The  term  habit  includes  the  form  and  general  appear- 
ance of  plants,  based  upon  stem,  branch,  and  leaf  characters. 
Thus,  trees  like  the  pines  and  spruces,  with  a  main  excurrent 
trunk,  are  said  to  be  erect  in  habit  as  compared  with  trees  like 
the  elm,  apple,  and  oak,  in  which  the  equal  growth  of  several 
branches  produces  a  spreading  habit.  Plants  are  also  said  to  be 
caulescent  when  possessed  of  a  definite  aerial  stem,  to  distinguish 
them  from  plants  like  dandelions  and  strawberries,  which  are 
designated  as  acaulescent,  or  without  a  visible  aerial  stem. 

Stems.  Stems  are  either  aerial,  growing  aboveground,  or  sub- 
terranean, growing  largely  or  wholly  underground.  The  main 
types  of  underground  stems  may  be  defined  as  follows : 

Rhizomes,  like  those  of  ferns  or  Solomon's  seal  (Fig.  207),  are 
horizontal  underground  stems  furnished  with  buds  and  scalelike 

353 


354  GENERAL  BOTANY 

leaves  which  serve  both  for  storage  and  for  the  support  of  aerial 
parts  growing  from  them.  On  account  of  their  underground 
habit  they  often  become  highly  modified  in  both  form  and 
structure. 

Runners  and  stolons  are  horizontal  stems  much  like  rhizomes 
except  that  they  run  over  the  surface  of  the  ground,  in  which 
they  frequently  take  root  and  give  rise  to  new  plants,  as  in  the 
strawberry  (Fig.  76).  Prostrate  stems  are  like  runners  except 

that  they  rarely  take  root 
in  the  soil  over  which 
they  trail. 

Bulbs  like  the  onion 
and  tulip  (Fig.  79)  are 
short,  erect  stems  which 
bear  scalelike  leaves  filled 
with  reserve  foods  sur- 
rounding a  terminal  bud. 

Corms  are  short,  erect, 
FIG.  207.   Rootstock  of  Solomon's  seal  .  ,„.       OAN 

fleshy    stems    (Fig.  80) 

&,  &,  buds;  r,  roots;  s,  flowering  stem  .  ,     .  .  , 

with  inconspicuous  scale 

leaves.  They  usually  have  a  prominent  terminal  bud  and  less 
conspicuous  lateral  buds  in  the  axils  of  the  scale  leaves. 

Tubers,  like  the  potato  (Fig.  78),  are  essentially  greatly 
shortened  rhizomes  with  scale  leaves  and  lateral  buds.  They 
are  usually  filled  with  stored  reserve  food. 

Leaves.  Fig.  208  illustrates  certain  characters  of  leaves,  per- 
taining to  their  form,  margin,  leaf  tips,  and  venation,  which  are 
of  importance  in  characterizing  and  identifying  plants.  The 
figures  are  self-explanatory,  since  the  proper  terms  descriptive 
of  the  leaves  are  used  in  connection  with  the  figures.  The 
terms  pinnate,  palmate,  and  parallel,  used  in  connection  with  the 
leaf  shapes,  indicate  the  type  of  venation  characteristic  of  each 
leaf.  Parallel  venation  is  characteristic  of  such  leaves  as  those  of 
the  linear  type,  found  in  grasses  and  members  of  the  lily  family, 
where  the  secondary  veins  run  lengthwise  of  the  leaf  parallel 
to  the  midvein.  Pinnate  and  palmate  venation  are  found  in  the 
broad-leaved  herbs  and  trees  which  belong  to  the  dicotyledons. 


FIG.  208.    Diagrams  illustrating  differences  in  the  form,  venation,  margin, 
and  apex  of  leaves 

a-k,  form  and  venation  of  leaves:  a,  lanceolate  pinnate;  6,  ovate  pinnate;  c,  heart- 
shaped  palmate ;  d,  halberd-shaped  palmate ;  e,  linear  parallel ;  /,  oblong  pinnate ; 
g,  oval  pinnate;  h,  orbicular  pinnate;  i,  oblanceolate  pinnate;  .;,  spatulate  pinnate; 
k,  obovate  pinnate,  l-r,  different  kinds  of  leaf  margins :  I,  serrate ;  m,  double  serrate ; 
n,  dentate ;  o,  crenate ;  p,  undulate ;  <?,  sinuate ;  r,  lobed.  s-w,  different  kinds  of 
leaf  apexes:  s,  acuminate;  t,  acute;  ?/,  obtuse;  v,  emarginate;  w,  mucronate 


356 


GENERAL  BOTANY 


FIG.  209.   Accessory  buds  of  box 
elder  (Negundo) 

A,  front  view ;  B,  two  groups  seen 
in  profile 


Buds.  Buds  may  be  classified 
on  the  basis  of  their  position  on 
the  main  axis  and  branches  or 
according  to  structure  and  devel- 
opment. On  the  basis  of  position, 
buds  are  terminal,  lateral,  axil- 
lary, accessory,  or  -adventitious. 

The  terms  terminal  and  lateral 
(or  axillary)  are  self-explanatory, 
since  such  buds  are  located  at 
the  apex  of  a  stem  or  branch  or 
in  the  axils  of  leaves. 

The  term  accessory,  or  super- 
numerary, applies  to  cases  in 
which  several  buds  arise  in  the 
axil  of  a  single  leaf,  as  in  maples 

(Fig.  209).    Adventitious  buds  arise  at  points  on  the  stem  other 

than  in  the  leaf  axils  and  are  usually 

the  result  of  an  injury. 

Buds  are  also  classified  on  the  basis 

of  structure  and  development.    Thus, 

branch  or  leaf  buds  develop  into  branches 

with   leaves;  flower  buds  form  single 

flowers,  or,   more  frequently,   inflores- 
cences, as  in  the  common  types  of  trees ; 

mixed  buds  give  rise  to  both  flowers  and 

shoots  with  leaves,  as  in  the  apple  and 

maple. 

Roots.    The  primary  root  is  the  first 

root  of   the  embryo  and  seedling.    In 

most    dicotyledons    the    primary    root 

forms  a  taproot  or  a  single  main  root, 

with  small  lateral  roots,  as  in  the  beet, 

carrot,  and  dandelion  (Fig.  210).    The 

primary  root  may  be  permanent  or  tem- 
porary, being  replaced  in  the  latter  instance  by  secondary  roots 

which  grow  out  from  the  lower  part  of  the  stem. 


FIG.  210.   Fleshy  taproot 
of  the  carrot 


DESCRIPTIVE  TERMS 


357 


Secondary  root  systems  take  their  rise  either  from  the  main  pri- 
mary root  or  from  the  underground  part  of  the  stem  (Fig.  211). 

Both  primary  and  sec- 
ondary roots  may  be 
either  fleshy  or  fibrous 
in  form,  and  surface  or 
deep,  according  to  their 
method  of  distribution 
in  the  soil. 

Adventitious  roots  arise 
from  stems  or  leaves 

and  are  thus  not  lateral 
FIG.  211.   Branching  root  system  of  corn 

branches  irom  a  primary 

or  a  secondary  root  system.  Adventitious  roots 
are  especially  common  in  the  monocotyledons. 
Roots  are  also  classified  according  to  the 
medium  in  which  they  grow ;  for  example, 
soil  roots,  water  roots,  and  air  roots  (or  aerial 
roots').  Air  roots  are  common  in  corn  as  prop 
roots,  growing  from  a  node  above  the  ground 
and  later  penetrating  the  earth  as  brace  roots. 

REPRODUCTIVE  PARTS  AND  PROCESSES 

Inflorescence.  The  inflorescence  has  already 
been  defined  as  a  cluster  of 
flowers,  to  distinguish  it  from 
solitary  flowers  borne  singly  in 
the  axils  of  ordinary  leaves 
(Fig.  212,  A).  Some  of  the 
main  types  of  inflorescence  are 
indicated  in  Fig.  213.  In  this 
figure  the  flowers  are  indicated 
by  circles  at  the  ends  of  the 
stems,  or  peduncles,  which  arise 
from  the  axils  of  bracts.  All  of  the  main  types  of  inflorescence  can 
be  derived  from  the  raceme  if  we  suppose  certain  modifications  to 


FIG.  212.    Solitary  flowers  and  cluster 

A,  axillary  and  solitary  flowers  of  the 

pimpernel;   E,  raceme  of  red  currant; 

p,  axis  of  inflorescence;  p',  peduncle; 

br,  bract 


358 


GENERAL  BOTANY 


have  arisen  in 'the  raceme  type  during  the  development  of  other 
kinds  of  inflorescence.  Thus,  the  spike  is  a  racemelike  inflores- 
cence in  which  the  numerous  flow- 
ers are  sessile,  without  peduncles. 
In  the  head  a  similar  modification 
in  the  peduncles  has  occurred, 
together  with  a  great  shortening 
and  thickening  of  the  axis  of 
inflorescence.  In  the  umbel  and 
corymb  the  axis  of  inflorescence 
is  short  and  the  peduncles  are  uni- 
formly lengthened,  so  as  to  bring 
the  flowers  at  the  same  level,  mak- 
ing a  flat-topped  inflorescence. 
Other  types  of  inflorescence  rep- 
resent similar  modifications  from 
the  simple  racemose  type. 

Flowers.    Flowers  have  various  terms  applied  to  them,  based 

upon  differences  in  form,  structure,  the  presence  or  absence  of 

the  perianth  or  essential  organs,  and  their  spiral  or  cyclic  plan. 

Cyclic  and  spiral  are 

— Petals' 
-Sepals' 

cavity- 


^ 
J     nnflore 


Head 


Umbel 


Corymb 


FIG.  213.    Diagram  illustrating 
various  types  of  inflorescence 


Hypogynous 


Pwigynous 


terms  used  to  desig- 
nate the  arrangement 
of  the  parts  of  flowers 
on  the  receptacle,  or 
axis.  Since  the  flower, 
as  already  indicated, 
is  a  modified  bud,  or 
branch,  with  the  parts 
corresponding  to  leaves, 
we  should  expect  the 
same  cyclic  or  spiral  ar- 
rangements of  its  floral 
parts  on  the  receptacle 
as  we  have  already  found  in  the  leaves  on  the  stem  axis. 


Epigynous 


l-JRcceptacle 
Hypog'ynous  Perigyndus  Epigynous 

FIG.  214.    Hypogynous,  perigynous,  and 

epigynous  flowers 
Diagrams  above  ;  median  sections  of  flower  below 


In 


our  discussion  of  the  flower  of  the  marigold  it  was  indicated 
that  such  flowers,  with  spirally  arranged  parts,  represented  the 


DESCRIPTIVE  TEEMS 


359 


simplest  floral  types,  comparable  to  the  strobilus  of  some  of  the 
lower  orders  of  plants.  In  the  cyclic  flowers,  like  the  mandrake 
and  bean,  the  floral  axis,  or  receptacle,  has  become  greatly  short- 
ened and  broadened,  so  that  the  floral  parts  are  arranged  on  the 
receptacle  in  cycles  instead  of  spirals  as  in  the  marigold.  The 
number  of  floral  parts  in  each  whorl  is  correspondingly  reduced 
in  cyclic  flowers  on  account  of  the  shortening  and  flattening  of 
the  receptacle.  With  these  modifications  in  the  receptacle  other 
changes  have 

been     gradually  /%'\         /?'\       ^Corolla 

introduced  with 
the  evolution  of 
flowers,  giving 
rise  to  what  are 
known  as  perig- 
ynous and  epig- 
ynous  flowers, 
as  distinguished 
from  the  more 
simply  arranged 
hypogynous  flow- 
ers (Fig.  214). 

Hypogynous  flowers  have  the  parts  of  the  perianth  and  the 
two  sets  of  essential  organs  (stamens  and  pistils)  arranged  sepa- 
rately on  the  receptacle.  The  spiral  flowers  of  the  marigold  and 
the  cyclic  flowers  of  the  mandrake  and  bean  are  common  illus- 
trations of  hypogynous  flowers  with  cyclic  and  spiral  arrange- 
ments of  their  floral  parts. 

Perigynous  flowers  have  a  cup-shaped  receptacle,  which  bears 
the  perianth  and  the  stamens  on  its  upper  margin.  These  parts 
thus  appear  to  surround  and  inclose  the  pistil,  which  remains 
free  in  the  center  of  the  flower.  Flowers  of  the  cherry  and  of  the 
common  wild  rose  are  familiar  illustrations  of  perigynous  flowers. 

Epigynous  flowers  are  common  in  the  apple  and  in  many 
flowers  of  the  Composite?,  to  which  the  yarrow  and  dandelion 
belong.  In  such  flowers  the  receptacle  is  cup-shaped,  as  in  perig- 
ynous flowers,  but  yet  forms  a  part  of  the  ovary  cavity,  which 


>  Stamen 
\- Pistil 


FIG.  215.    Complete  flower  of  the  alpine  azalea 
(Loiseleuria) 

A,  exterior  view ;  B,  sectional  view.   After  H.  Miiller 


360 


GENERAL  BOTANY 


is  roofed  over  by  the  sporophyll  or  sporophylls  of  the  pistil 
proper.  The  remaining  parts  of  the  epigynous  flower  seem  to 
arise  from  the  upper  margin  of  the  ovary,  surrounding  the  style 
and  stigma. 

Flowers  are  also  variously  classified  on  the  basis  of  the  pres- 
ence or  absence  of  certain  floral  parts  and  of  the  form  of  floral 
parts,  and  also  on  the  basis  of  certain  arrangements  for  insuring 

pollination  by  wind  or 
insects.  The  following  are 
the  main  classes  which  the 
student  may  be  expected 
to  meet  in  an  elementary 
course. 

Complete  flowers  have  all 
four  sets  of  floral  organs 
(calyx,  corolla,  stamens, 
and  pistil)  represented  in 
one  flower,  while  incomplete 
flowers  have  one  or  more 
sets  lacking. 

Perfect  flowers,  like  the 
mandrake  and  marigold, 

a  willow  (Salix  alba)  have  both  sets  of  essential 

a,  staminate  catkin;  6,  pistillate  catkin ;  c,stam-      Organs    (stamens    and    pis- 
inate  flower ;  d,  pistillate  flower.  From  Bergen      tQs^    present,   while   oper- 
and Cald  well's  "  Practical  Botany  "  *'•  i  .    j 

feet  flowers  lack  one  kind 

of  essential  organs  and  are  thus  either  wholly  staminate  or 
wholly  pistillate.  Plants  which  bear  imperfect  flowers  are  said 
to  be  either  monoecious  or  dioecious,  according  as  they  bear  imper- 
fect flowers  of  both  kinds  on  the  same  or  on  different  plants. 
Thus,  the  willows  (Fig.  216)  are  imperfect  and  dioecious,  since 
they  have  only  staminate  flowers  on  one  tree  and  only  pistillate 
flowers  on  another  tree  of  the  same  species.  The  oak,  on  the 
contrary,  is  imperfect  and  monoecious,  with  both  staminate  and 
pistillate  flowers  on  the  same  tree  (Fig.  238). 

Regular  flowers  have  all  of  the  parts  of  one  set  of  organs 
alike  in  form,  as  in  the  azalea,  while  irregular  flowers  have  the 


FIG.  216.   Catkins  and  dioecious  flowers  of 


DESCRIPTIVE  TEEMS 


361 


parts  of  one  or  more  sets  of  organs  irregular  in  form,  as  in  the 
flowers  of  the  pea,  bean,  and  locust  (Fig.  89). 

Floral  plan.  In  a  floral  plan  (Fig.  217>the  parts  of  the  flower 
are  represented  in  transverse  section  as  though  reduced  to  a 
common  plane.  In  dicotyledons  the  parts  are  usually  in  fours  or 
fives,  while  in  monocotyledons  they  are  on  the  plan  of  three.  It 
will  be  noticed  also  that  the  parts  of  each  set  of  floral  organs 
alternate  with  those  adjacent  to  them,  like  the  leaves  on  a  leafy 
shoot.  This  alternate  arrangement  is  another  evidence  that 
flowers  are  modified  shoots. 

Pollination  features.  The  special  structural  and  physiological 
phenomena  concerned  with  the  pollination  of  flowers  relate  for 


b  c 

FIG.  217.   Floral  diagrams 

a,  lily  family;  b,  heath  family;  c,  madder  family;  d,  composite  family.    The  dot 

above  b  and  d  indicate  the  stem  axis ;  the  sepals  are  represented  with  midribs ;  the 

lighter  stamens  in  b  represent  an  alternate  whorl  of  stamens.   After  Sachs 

the  most  part  to  devices  for  securing  cross-pollination  and 
close-pollination.  The  more  important  of  these  structural  and 
physiological  phenomena  are  the  following: 

Anemophilous  and  entomophilous.  The  principal  agents  by 
which  flowers  are  close-pollinated  or  cross-pollinated  are  the  wind 
and  insects.  Flowers  in  which  the  pollen  is  carried  to  the  stigma 
by  the  wind,  as  in  corn,  poplar,  and  oaks,  are  said  to  be  ane- 
mophilous,  or  ^  wind-loving,"  while  insect-pollinated  flowers,  like 
the  locust  and  bean,  are  said  to  be  entomophilous,  or  "  insect- 
loving."  Anemophilous  flowers  are  usually  characterized  by  in- 
conspicuous color,  abundance  of  light  pollen,  and  lack  of  odor. 

Dichogamy.  In  many  perfect  flowers  the  stamens  and  stigma 
mature  at  different  dates  in  the  same  flower,  —  a  condition 
defined  by  the  term  dichogamy  (Fig.  218).  If  the  stamens  ripen 
earlier  than  the  stigmas  in  such  flowers,  the  flowers  are  said  to 


362 


GENERAL  BOTANY 


be  protandrom,  while  flowers  in  which  the  stigmas  ripen  before 

the  anthers  are  said  to  be  protogynous.    It  is  evident  that  protan- 

drous  and  protogynous  flowers  are 
necessarily  either  close-pollinating 
or  cross-pollinating.  Homogamous 
flowers  are  flowers  in  which  the  sta- 
mens and  pistils  ripen  together,  thus 
making  self-pollination  possible. 

It  is  evident  that  where  flowers 
are  imperfect  either  close-pollination 
or  cross-pollination  is  insured,  since 
self-pollination  would  be  impossi- 
ble in  such  flowers. 

Heterostylous  flowers.  Flowers 
in  which  the  stamens  and  styles 
are  of  different  lengths  are  said 
to  be  heterostylous.  Heterostylous 
flowers  may  be  either  dimorphic 
(Fig.  219)  (with  two  lengths  of 
stamens  and  pistils)  or  trimorpJiic 

(with   three    lengths   of   stamens  and   pistils).    In   either  'case 

each   set   of   stamens   matches    one    length   of    pistils,    so   that 

insects  carry  pollen  from  the  anthers  of  one  flower  to  stigmas 

of  the    same   height 

in   other   flowers    on 

the    same    or    on   a 

different  plant,  thus 

effecting      either     a 

close-pollination  or  a 

cross-pollination. 
Odor,  nectar,  color, 

and  movements.     In 

flowers     adapted    to 

insect  pollination,  or 

entomophily,   the   insects    are   undoubtedly   attracted  to   many 

flowers  by  their   odor,   nectar,   or   color,   a  fact  which  insures 

close-pollination  and  cross-pollination.    The  odor  is  due  to  the 


FIG.  218.    Dichogamy  in  flowers 
of  Clerodendron 

a,  the  pistil  is  hent  to  one  side  away 
from  the  ripe  stamens  in  the  young 
flower;  6,  in  older  flowers  the  sta- 
mens wither  and  the  stigmas  are 
exposed  for  the  reception  of  pollen 


a 


A  B 

FIG.  219.  Dimorphic  stamens  and  pistils  in  bluets 

A,  form  with  long  style ;  13,  form  with  short  style ; 
a,  anthers;   s,  stamens 


DESCKIPTIVE  TEEMS 


363 


secretion  of  volatile  oils  by  the  petals  or  other  floral  parts,  while 
the  nectar  is  secreted  by  nectar  glands,  usually  located  at  the 
base  of  the  pistils  on  the  receptacle.  In  addition  many  flowers 
possess  the  power  of  movement  in  the  stamens  and  pistils 
by  which  the  anthers  and  stigmas  are  either  separated  or 
approximated  when  ripe,  thus  insuring  either  close-pollination, 
cross-pollination,  or  self-pollination. 

Pistils,  seeds,  and  fruits.  Pistils  are  either  simple  or  com- 
pound, according  as  they  are  composed  of  one  or  more  spo- 
rophylls,  or  carpels.  Simple  pistils  are  composed  of  one  carpel, 


— 'Stigma" 
Carpel^ 


Placenta 
—Ovary 


FIG.  220.    Simple  and  compound  pistils 

A,  simple  pistil  with  one  carpel;  B,  compound  pistil  with  two  carpels  and  central 
placenta ;  C,  compound  pistils  (a,  with  parietal  placentae ;  b,  c,  with  central  placenta) 

or  megasporophyll,  as  in  the  mandrake,  bean,  and  locust 
(Fig.  220,  A).  Compound  pistils  are  composed  of  two  or  more 
carpels,  or  sporophylls,  so  united  as  to  inclose  one  or  more  seed 
cavities,  or  locules  (B).  The  placentae,  or  lines  of  attachment  of 
the  ovules,  may  be  either  central  or  parietal  (.C'). 

Ovules  are  of  three  main  types,  according  to  their  form  and 
the  relation  of  the  ovule  proper  to  the  funiculus.  Orthotropous, 
or  straight,  ovules  grow  straight,  without  curvature,  from  the 
funiculus,  or  stalk.  Campylotropous  ovules  are  curved,  owing 
to  the  greater  growth  of  one  side  of  the  ovule  during  its 
development,  as  in  Capsella  (Fig.  203,  (7).  Anatropous  ovules 
are  the  most  common  type,  in  which  the  ovule  becomes  com- 
pletely inverted  during  its  early  development  and  adheres  to 


364 


GENERAL  BOTANY 


the  funiculus  throughout  its  entire  length.    The  ridgelike  junc- 
tion of  the  ovule  and  the  funiculus  is  called  the  raphe. 

Fruits  are  usually  formed   as  a  result  of  fertilization,  and 
consist  of  the  ripened  ovary  or  of  the  ovary  and  the  receptacle, 


.\  Follicle  .^ 
(one  carpel) 
Capsule 

(three  carpels) 


Silique  Capsule 

(two  carpels)  (four  carpels) 


Legume 
larpel      (one  carpel) 


Achenes   Single  achene 

(buttercup) 


Samara,  or 
key  fruit 
(elm) 


Samara,  or  key 
fruit  (maple) 


Berry 

(Smilacina)   Aggregate 
(mulberry) 


Pome  (apple) 


Drupe,  or 
stone  fruit 
(peach) 


FIG.  221.    Different  kinds  of  fruits 

Upper  row,  dry  dehiscent  fruits ;  middle  row,  dry  indehiscent  fruits ;  lower  row, 

fleshy  fruits 

as  in  the  pome,  drupe,  and  aggregate  fruits  (Fig.  221).  They 
are  variously  classified  on  the  basis  of  their  form  and  structure. 
In  the  following  classification  the  terms  dry  and  fleshy  indicate 
whether  the  ovary  (or  the  ovary  and  receptacle  combined) 


DESCRIPTIVE  TERMS  365 

becomes  dry  and  hard  in  ripening,  like  the  fruit  of  the  pea  and 
buttercup,  or  soft  and  fleshy,  as  in  the  apple,  pear,  etc. 

Dehiscent  fruits,  upon  ripening,  split  open  along  the  junction 
of  the  carpel  or  carpels,  namely,  along  the  placenta  (septicidal 
dehiscence)  or  along  the  back  of  the  carpels  between  the  pla- 
cental  junction  line  (loculicidal  dehiscence). 

Indehiscent  fruits  remain  closed,  as  in  the  cereal  grains  and 
strawberry,  where  the  entire  ovary  and  inclosed  ovule  is  shed 
and  disseminated  together. 

The  following  classification  and  the  accompanying  illustra- 
tions will  enable  the  student  to  classify  most  of  the  common 
fruits  with  which  he  comes  in  contact  in  the  field. 

Dry  dehiscent  fruits : 

The  follicle  is  a  simple  fruit  which  dehisces  along  one  side. 
The  legume,  or  pod,  is  a  simple  fruit  which  dehisces  along  two 

sides,  as  in  the  bean  and  pea. 
The  silique  is  a  fruit,  like  that  of  mustard,  composed  of  two  spo- 

rophylls,  or  carpels,  which  separate  from  the  central  partition. 
The  capsule  is  a  fruit  formed  from  a  compound  ovary  which 

opens  at  the  junction  of  the  sporophylls  or  between  these 

junction  points. 
Dry  indehiscent  fruits : 

The  achene  is  a  simple  dry  fruit  in  which  the  single  seed  is  free 

from  the  ovary  wall,  as  in  the  buttercup. 
The  caryopsis,  or  grain,  is  the  fruit  of  the  grasses  and  cereals 

in  which  the  ovary  wall  adheres  to  the  seed. 
The  nut  is  a  fruit  in  which  the  ovary  wall  becomes  the  indurated 

resistant  wall  of  the  fruit. 
The  samara,  or  key  fruit,  like  that  of  the  maple  and  ash,  is  a 

fruit  furnished  with  a  winglike  outgrowth  of  the  ovary  wall. 
Fleshy  fruits,  simple  or  compound  : 

In  the  berry  the  ovary  wall  becomes  fleshy  and  incloses  one  or 

more  seeds. 
In  the  pome  the  ovary  wall  is  fleshy  but  with  an  indurated 

central  part  inclosing  the  seeds,  as  in  the  core  of  the  apple. 
Drupes  are  stone  fruits,  like  the  cherry  and  plum. 
Aggregate  fruits,  like  the  blackberry,  have  several  simple  stone 

fruits  aggregated  or  massed  together  on  one  receptacle. 


CHAPTER  XIX 

TREES,  SHRUBS,  AND  FORESTS 
IMPORTANCE  AND  USE 

Ornament  and  protection.  The  ornamental  and  protective 
function  of  trees  is  so  well  known  that  very  little  can  be  said 
to  emphasize  this  aspect  of  their  importance  to  man.  The 
shade  trees  of  our  cities  and  towns,  the  great  beauty  of  trees 
and  shrubs  on  private  lawns  and  in  public  parks,  the  pictur- 
esqueness  of  the  mountains  and  of  the  open  country  with 
wooded  hills  and  streams,  all  attest  to  the  value  of  trees  and 
shrubs  and  to  the  need  of  an  adequate  knowledge  of  their 
habits  and  uses. 

These  facts  are  more  evident  if  one  travels  from  the  wooded 
regions  of  the  East  or  the  Far  West  across  the  Western 
prairies,  where  little  protection  is  offered  against  wind,  sun, 
and  storm  except  where  early  settlers  have  established  wind- 
breaks by  setting  out  trees  or  where  streams  are  bordered  by 
protective  stands  of  timber.  While  isolated  trees,  or  trees  in 
small  groups,  are  thus  contributory  to  man's  pleasure  and  com- 
fort, it  is  to  trees  aggregated  in  forests  that  one  must  turn  in 
order  to  understand  the  great  importance  of  tree  life  to  the 
industrial  life  of  men  and  to  the  progress  of  civilization. 

The  national  forests.  The  national  forests  of  the  United  States 
formerly  occupied  an  area  of  850,000,000  acres,  which  has 
been  reduced  at  the  present  time  to  about  545,000,000  acres. 
This  vast  forest  domain  includes  the  northern  coniferous  forests 
of  the  Great  Lakes  and  the  mixed  coniferous  and  hardwood  for- 
ests of  the  New  England  States ;  the  great  southern  forests, 
composed  largely  of  pines ;  the  central,  sparsely  covered  forest 
of  hard  woods;  and  the  Rocky  Mountain  and  Coast  Range 
forests  of  the  extreme  western  states  (Fig.  223). 

366 


TBEES,  SHRUBS,  AND  FOEESTS  367 

The  great  value  of  trees  in  this  forest  domain  is  enhanced 
by  the  fact  that  a  large  part  of  it  is  in  the  mountains  and  in 
regions  like  the  pine  barrens  of  the  Southern  states,  where 
the  land  is  not  of  value  for  agriculture  on  account  of  the  un- 
productiveness of  the  soil  in  those  regions.  The  trees  thus 
render  an  otherwise  unfruitful  region  productive,  and  serve 
at  the  same  time  as  a  protection  against  floods,  erosion,  and 
drought  by  their  control  of  rainfall  and  other  climatic  factors. 


FIG.  222.   The  ornamental  function  of  trees  and  shrubs 
Photograph  furnished,  by  the  United  States  Forest  Service 

Climate  and  water  supply.  The  factors  of  climate  which  are 
controlled  in  any  measure  by  forests  are  concerned  largely  with 
temperature,  air  movements,  and  water  control.  The  effect  on  tem- 
perature is  one  which  is  felt  only  in  the  immediate  vicinity 
of  the  forests  themselves,  and  not  over  the  country  at  large. 
It  is  a  well-known  fact  that  the  leaves  of  trees  in  a  forest 
absorb  a  large  part  of  the  heat  which  falls  upon  them,  and 
that  they  utilize  this  heat  in  warming  the  leaves,  in  making 
sugar  and  starch,  and  in  the  evaporation  of  water  vapor.  The 
rich  covering  of  humus  on  the  forest  floor  also  absorbs  heat 
and  protects  the  soil  beneath  from  absorbing  and  radiating  it 
as  the  soil  in.  naked  exposed  regions  would.  As  a  consequence 


368 


GENERAL  BOTANY 


the  forest  has  a  general  cooling  effect  on  the  air  in  its  vicinity 
and  so  protects  'the  soil  from  drying  up.  Air  movements  in  the 
form  of  winds  and  storms  are  also  restricted  by  forests,  which 
therefore  serve  as  effective  windbreaks  and  at  the  same  time 
affect  the  temperature  of  a  region  in  both  winter  and  summer. 
The  early  settlers  in  the  West  soon  learned  this  advantage  of 
trees  as  windbreaks  and  planted  cottonwoods  and  other  quick- 
growing  trees  on  the  north  and  west  sides  of  their  holdings. 


1  Eastern  Region  \.%  • 

2  Central  Treeless  Region 

3  Western  Region 


TROPICAL 
FOREST 


FIG.  223.    General  map  of  the  forest  areas  of  the  United  States 

In  addition  to  the  effect  on  the  relative  humidity  and  tem- 
perature of  a  region  the  forests  have  an  important  function  in 
the  control  of  water  falling  in  the  form  of  rain  or  snow. 

Forest  control  of  rainfall  and  floods.  Rainfall  is  supposed  by 
some  to  be  increased  by  the  presence  of  great  forests,  and  the 
investigations  of  European  foresters  would  seem  to  bear  out 
this  assumption.  Other  data,  however,  gathered  with  equal 
care  by  experienced  scientific  foresters,  yield  opposite  results, 
and  it  is  doubtful  whether  the  forests  have  any  marked  effect 
on  precipitation. 

Floods  and  erosion,  or  the  wearing  away  of  soil  by  water, 
are  so  largely  controlled  by  forests  that  this  control  is  now 


TREES,  SHKUBS,  AND  FORESTS 


369 


reckoned  among  the  most  beneficent  and  important  effects  of 
the  forest  cover.  When  rain  falls  over  a  dense  forest,  from  one 
tenth  to  one  fourth  of  it 
is  caught  by  the  crowns 
of  the  trees,  while  the 
forest  floor  of  humus  soil 
and  roots  holds  the  re- 
mainder. It  is  estimated 
that  a  forest  floor  "  can 
hold  for  a  while  a  rain- 
fall of  five  inches."  This 
water  is  then  gradually 
evaporated  or  is  slowly 
drained  off  into  streams, 
lakes,  and  the  sources 
of  springs.  Mountain 
streams,  which  irrigate 
fertile  valleys,  are  thus 
fed  and  sustained  at  their 
source.  In  like  manner 
the  supply  of  water  for 
the  great  irrigation  sys- 
tems of  the  West,  and 
for  the  water  supplies  of 
large  cities  like  Denver, 
Los  Angeles,  and  San 
Francisco,  comes  from 
mountain  springs,  lakes, 
and  streams,  which  are 

protected  at  their  source 

FIG.  224.    Pacific-coast  forest  of  Douglas  fir 
and  western  red  cedar,  Tacoma,  Washington 


Photograph  furnished  hy  the  United  States 
Forest  Service 


by  forests. 

Rain  which  falls  upon 
unf  orested  soil  has  a  very 
different  effect  from  that 

outlined  above,  especially  in  mountainous  and  hilly  regions, 
such  as  those  occupied  by  most  of  our  national  forests.  In  a 
region  denuded  of  forests  the  rain  falls  directly  upon  the  soil, 


370  GENERAL  BOTANY 

which  is  apt  to  be  beaten  into  a  hard  surface  layer  or,  if  soft  and 
porous,  to  become  quickly  saturated  and  give  way.  The  result 
is  almost  certain  to  be  a  disastrous  flood  and  often  immense 
damage  caused  by  greatly  swollen  streams.  In  the  Adirondacks 
and  in  California  great  damage  has  already  been  done  in  this 
way  where  the  forests  have  been  wholly  or  partially  cut  off  or 
injured  by  grazing.  It  is  estimated  that  "  upward  of  two  hundred 
square  miles  in  the  United  States  is  annually  laid  waste  by 
erosion  "  and  that  much  of  this  great  waste  could  be  prevented 
by  protecting  or  replanting  the  forests  in  the  eroded  regions. 
When  swollen  mountain  streams  reach  the  valleys  at  the  foot 
of  the  mountains,  they  flood  them  and  at  the  same  time  deposit 
sand,  gravel,  and  even  large  bowlders  on  once  fertile  and  pro- 
ductive soil.  Not  only  must  the  cutting  of  forests  on  mountain 
slopes  be  carefully  regulated,  therefore,  but  denuded  areas  need 
to  be  systematically  reforested  by  governments,  either  state  or 
national,  which  possess  resources  adequate  for  such  great  tasks. 

Forest  products.  A  great  variety  of  forest  products  are  derived 
from  the  national  forests,  including  turpentine,  tar,  formalde- 
hyde, and  rosin,  in  addition  to  the  more  important  wood  pulp, 
timber,  and  lumber  supplies.  The  lumber  and  timber  are  used 
for  various  purposes  in  the  industries  and  the  home.  These 
uses  include  firewood,  lumber  for  construction  and  building, 
cooperage,  and  veneers,  and  timber  for  the  making  of  excelsior 
and  wood  pulp,  railroad  ties,  and  telephone  and  telegraph 
poles.  For  these  various  purposes  it  is  estimated  that  "  we  take 
from  our  forests  yearly,  including  waste  in  logging  and  manu- 
facture, more  than  22,000,000,000  cubic  feet  of  wood,  valued 
at  $1,375,000,000."  It  is  also  estimated  that  almost  half  of  the 
original  lumber  supply  of  the  United  States  has  already  been 
used  and  that  "the  present  rate  of  cutting  for  all  purposes 
exceeds  the  annual  growth  of  the  forests." 

The  remedy.  The  obvious  remedy  for  this  condition  is  the 
scientific  control  of  timber  cutting  and  the  replanting  of  the 
forests,  already  in  process  of  depletion,  by  the  state  and  national 
governments.  It  is  therefore  of  the  greatest  importance  that  the 
United  States  government  has  adopted  the  policy  of  caring  for 


TREES,  SHRUBS,  AND  FORESTS  371 

and  extending  its  control  over  an  ever-increasing  forest  area. 
The  extent  to  which  this  policy  is  being  carried  out  by  our 
national  government  is  indicated  by  the  following  data.  "  On 
June  30,  1917,  there  were  147  national  forests  with  a  total  of 
155,166,619  acres,"  yielding  an  annual  income  of  $3,500,000. 
On  the  above  date  the  government  employed  in  this  work  between 
three  and  four  thousand  men,  including  forest  supervisors  and 
rangers,  lumbermen,  sealers,  planters,  and  clerks.  These  various 
officers  are  distributed  to  the  different  national  forests  in  the  pro- 
portion necessitated  by  the  labor  to  be  performed.  They  have 
a  great  variety  of  work,  including  the  prevention  and  control  of 
forest  fires,  the  scientific  cutting  and  marketing  of  timber,  the 
control  of  grazing  privileges,  and  the  replanting  of  depleted 
forests.  This  mere  enumeration  of  the  extent,  use,  and  control 
of  the  national  forests  of  the  United  States  is  all  that  can  be 
attempted  in  an  elementary  textbook  of  botany,  but  every  student 
should  acquaint  himself  with  this  great  industry  of  our  national 
government,  which  means  so  much  to  the  present  and  future 
prosperity  of  our  country. 

REPRESENTATIVE  GROUPS  OF  FOREST  TREES 

The  trees  which  comprise  the  forests  of  the  United  States 
belong  to  the  gymnosperms,  or  naked-seeded  plants,  represented 
by  the  pines  and  the  spruces,  and  to  the  angiosperms  with  a 
closed  pistil,  represented  by  the  elm,  oak,  and  maple.  In  the 
following  species,  selected  from  these  two  great  tree  groups, 
both  the  economic  and  the  biological  features  will  be  considered 
as  concrete  illustrations  of  the  importance  and  interest  attached 
to  forest  and  ornamental  trees. 

GYMNOSPERMS  (EVERGREENS) 
THE  SPRUCES  (PiCEA) 

Habitat  and  habit.  The  spruces  form  an  important  part  of  the 
great  coniferous  (cone-bearing)  and  mixed  forests  of  the  north- 
eastern portion  of  the  United  States,  the  Appalachian  region,  the 


372 


GENERAL  BOTANY 


Rockies,  and  the  North  Pacific  coast.  The  red,  white,  and  black 
spruces  are  found  mainly  in  the  northeastern  and  Appalachian 
forests ;  the  Engelmann  spruce  has  its  home  in  the  Rocky  Moun- 
tains; while  the  Sitka  spruce,  so  important  in  the  construction 
of  aeroplanes  in  the  World  War,  is  found  exclusively  on  the 

Pacific  coast.  The  hab- 
itat of  the  spruces  is 
thus  confined  largely 
to  well-drained  up- 
lands or  to  mountain 
slopes.  Like  many 
other  plants,  spruces 
are  often  found  to  be 
occupying  situations 
to  which  they  are  not 
perfectly  adapted,  in- 
cluding marshes  and 
swamps,  on  account 
of  the  lack  of  compe- 
tition in  these  habitats 
with  the  more  highly 
organized  hard  woods, 
such  as  the  oak  and 
maple. 

Tolerance.  An  im- 
portant factor  in  the 
distribution  and  suc- 
cess of  the  spruces 
is  due  to  the  light 

requirement  of  the  different  species,  especially  in  the  younger 
stages  of  growth.  They  belong  to  the  so-called  tolerant  trees, 
which  have  the  power  to  grow,  while  young,  in  the  shade  of 
other  trees  (Fig.  225).  "  Having  once  gained  a  foothold  in  a 
selection  forest,  the  young  spruce  grips  life  tenaciously,  strug- 
gles along  for  many  years  under  the  shade  of  the  forest,  and 
gradually  forces  its  way  upward  as  natural  thinning  reduces  the 
number  of  its  overtopping  competitors."  Balsam  is  often  found 


FIG.  225.   Virgin  stand  of  red  spruce  with  repro- 
duction of  tolerant  spruce  and  fir  in  the  White 

Mountains,  New  Hampshire 
Photograph  by  the  United  States  Forest  Service 


TREES,  SHRUBS,  AND  FORESTS 


373 


with  spruce  on  the  forest  floor  (Fig.  226),  since  it  too  is  a  tolerant 
species,  growing  in  the  shade  of  the  other  forest  trees.  Balsam 
is,  however,  the  stronger  competitor  of  the  two  in  such  situa- 
tions, on  account  of  its  more  plentiful  seeds  and  rapid  growth. 
Seed  production.  The  distribution  of  forest  trees,  and  their 
power  to  reproduce  a 
forest  once  destroyed, 
is  determined  largely 
by  the  number  of 
seeds  produced  and 
by  the  viability  of 
the  seeds,  or  their 
power  to  germinate 
and  grow  under  the 
conditions  presented 
in  a  given  habitat. 
Spruces,  like  other 
cone-bearing  gymno- 
sperms,  begin  to  pro- 
duce large  quantities 
of  winged  seeds  when 
the  trees  have  reached 
the  proper  age.  The 
seed-producing  stage 
has  been  found  to 

•^  FIG.  226.  Balsam,  a  tolerant  tree,  growing  beneath 

with  the  conditions  a  virgin  stand  of  red  spruce  in  the  White  Mountains 
Under  which  it  lives.  Photograph  by  the  United  States  Forest  Service 

In  the  forest  it  begins 

to  bear  when  the  crown  succeeds  in  reaching  the  light,  which 
may  be  at  the  age  of  twenty  or  thirty  years  or  may  be  delayed 
until  the  tree  is  one  hundred  years  old.  "In  the  open,  and 
under  favorable  soil  conditions,  seed  production  begins  as  early 
as  the  fifteenth  or  twentieth  year,  and  heavy  crops  follow  by 
the  thirtieth  or  thirty-fifth  year."  The  seeds  mature  in  late 
September  and  germinate  in  the  same  fall  or  the  next  spring, 
producing  in  good  soil  a  new  stand  of  spruce. 


374 


GENERAL  BOTANY 


Maintaining  the  supply.  Maintaining  the  supply  of  spruce  for 
wood-pulp  production  and  other  commercial  purposes  is  closely 
connected  with  the  amount  and  nature  of  the  seed  production, 
since  spruce  forests  are  recreated  or  regenerated  largely  by 
means  of  seeds.  In  the  case  of  hardwood  trees,  to  be  discussed 
below,  vegetative  reproduction  by  means  of  sprouts  from  the 
stumps  is  often  used  hi  the  regeneration  of  a  forest  destroyed 

by  cutting  or  by  other 
agencies.  In  the  case  of 
most  of  the  coniferous 
trees  the  sprout  method 
is  not  possible,  since,  with 
few  exceptions,  these  trees 
do  not  reproduce  vege- 
tatively  in  this  manner. 
The  production  of  new 
stands  of  spruce  by  means 
of  seeds  may  be  either 
by  the  natural  method, 
where  the  growth  of  seed- 
lings occurs  in  a  spruce  or 
a  mixed  forest  (Fig.  229), 
or  by  artificial  sowing  of 
seed.  Where  seedlings 
are  to  be  grown  by  the 
natural  method,  care  must  be  taken  to  cut  out  enough  of  the 
standing  timber  to  facilitate  the  growth  of  the  spruce  at  each 
stage  of  its  development.  In  time,  most  or  all  of  the  larger  trees 
of  such  an  area  will  need  to  be  cut,  to  allow  the  new  spruce 
forest  to  develop  normally,  with  plenty  of  soil  space  and  light 
exposure.  In  other  instances  clear  spaces  are  cut  in  the  forest, 
with  bordering  mother  spruce  trees,  from  which  the  seed  will  be 
distributed  and  sown  naturally  over  the  cleared  ground.  In  such 
cases  the  surrounding  trees,  if  the  clearing  is  not  too  large,  pro- 
tect the  ground  from  drying  and  furnish  partial  protection  to  the 
growing  seedlings.  These  sheltering  trees  must  be  allowed  to 
stand  until  the  young  growth  can  bear  direct  exposure. 


FIG.  227.   Ripe  cones  of  big-cone  spruce  in 
the  Cleveland  National  Forest,  California 

Photograph  by  the  United  States  Forest  Service 


TREES,  SHRUBS,  AND  FORESTS 


375 


Seeding  with  spruce  seeds  may  also  be  done  artificially  by 
scattering  seeds  on  soil  denuded  of  forest  trees  or  by  sowing  the 
seeds  in  prepared  seed  beds  (Figs.  230  and  231).  In  this  case 
the  seedlings,  when  they  have  reached  the  desired  age,  must  be 
transplanted  to  the  forest  area  where  the  new  forest  is  to  be 
grown.  This  method 
is,  on  the  whole,  the 
best  and  will  prob- 
ably be  more  largely 
employed  in  the  future 
than  in  the  past  by 
the  state  and  national 
governments.  Valu- 
able species  for  this 
purpose  are  the  white 
spruce  (Picea  canaden- 
sis)  and  the  Norway 
spruce  (Picea  abies), 
while  the  red  spruce 
(Picea  rubra)  is  more 
difficult  to  manage  on 
account  of  its  slow 
growth  in  early  life. 

Commercial  impor- 
tance. The  wood  of 
the  spruce,  like  that 
of  the  pines  and  of 
the  other  cone-bearing 
trees,  belongs  to  the 
class  called  softwood, 
to  distinguish  it  from  that  of  the  broad-leaved  hardwood  trees 
like  the  oak,  maple,  hickory,  and  poplar.  The  term  is  a  purely 
conventional  one,  since  many  kinds  of  soft  woods  are  harder 
and  more  durable  than  some  of  the  so-called  hard  woods,  like 
poplar,  basswood,  and  willow.  The  real  characteristic  of  spruce 
and  other  coniferous  woods  which  gives  them  their  value  and 
distinguishes  them  from  the  wood  of  broad-leaved  species  is  the 


FIG.  228.  Reproduction  of  spruce  (second  growth) 
in  New  Hampshire 

Photograph  hy  the  United  States  Forest  Service 


376 


GENERAL  BOTANY 


character  of  the  wood  elements  which  make  up  the  bulk  of  the 
wood.  The  student  will  recall  that  in  spruce  wood  (Fig.  193) 
the  water-carrying  elements  were  the  one-celled,  thick-walled 
tracheids  instead  of  the  wide  ducts  of  the  alder  and  other  hard 
woods.  The  small  diameter  of  these  tracheids,  their  thick  walls, 
and  their  uniform  size  throughout  the  tree  trunk  make  the 
even,  fine-grained  wood  of  the  spruces  and  other  conifers, 
like  the  white  pine,  extremely  valuable  in  the  industries. 


FIG.  229.   Regrowth  of  aspen  and  spruce  on  a  burned  area  in  the  San  Francisco 
Mountains,  Arizona 

Photograph  furnished  by  the  United  States  Forest  Service 

The  long,  fibrous  character  of  these  tracheids  and  their  close 
union  with  each  other  also  contribute  to  their  great  value  in 
the  wood-pulp  industry  and  in  the  making  of  aeroplanes  from 
the  now  famous  Sitka  spruce.  "  Spruce  is  an  aristocrat  among 
woods.  Its  outstanding  characteristics  are  combined  elasticity 
and  the  ability  to  withstand  sudden  strain  and  shock." 

The  most  extensive  use  of  spruce  at  the  present  time  is  in 
the  making  of  wood  pulp  for  newspaper  stock.  Something 


FIG.  230.    State  nursery  at  Saranac,  Adirondack  Mountains,  New  York 

Two-year-old  seedlings  of  yellow  pine  in  the  foreground.   Photograph  furnished  by 
the  United  States  Forest  Service 


FIG.  231.   Nursery  of  Austrian  and  yellow  pine  in  the  Kansas  National  Forest 

Austrian  pine  on  the  right,  yellow  pine  on  the  left.    Photograph  furnished  by  the 

United  States  Forest  Service 

377 


378  GENERAL  BOTANY 

over  four  million  cords  of  wood  is  used  for  this  purpose 
annually,  of  which  about  60  per  cent  has  been  spruce  wood. 
Red  spruce  has  been  the  principal  contributor  to  this  great 
enterprise,  but  other  woods  are  now  being  used  on  account  of 
the  depletion  of  the  American  and  Canadian  forests  in  the  trees 
of  this  species.  Spruce  wood  is  also  widely  used  in  slack  cooper- 
age and  in  building  and  interior  finishing.  The  great  value  of 

spruce  wood  in  the 
industries  has  stim- 
ulated the  govern- 
ment to  investigate 
new  methods  for 
its  preservation  and 
regeneration  in  the 
forests,  which  will 
undoubtedly  result 
in  preserving  these 
valuable  trees  to 
future  generations. 

THE  PINES 

Habitat  and  habit. 

FiG.232.  Transversesectionof  tree  trunk  of  long-leaf      Th       .         originally 
pine,  showing  annual  rings,  heartwood,  and  sapwood  .    ®          J 

Photograph  furnished  by  the  United  States  Forest  Service      occurrecl 

mixed  and  pure  for- 
ests in  the  northern,  southern,  and  western  national  forests. 
The  great  stand  of  pure  pine  in  the  northern  forests  in  the 
Great  Lakes  region  has  been  almost  wholly  depleted,  however,  so 
that  the  southern  forest  of  long-leaf  and  short-leaf  pine,  loblolly 
pine,  and  cypress  is  one  of  the  principal  sources  of  pine  lumber 
to-day.  This  is  also  the  great  seat  of  the  turpentine  industry, 
which  has  exacted  a.  heavy  toll  on  southern  pines  under  the 
old  wasteful  system  of  tapping  the  trees  for  turpentine.  The 
Western  Rocky  Mountain  and  Pacific  Coast  forests  also  supply 
pine  lumber  in  large  quantities  from  the  western  yellow,  lodge- 
pole,  sugar,  and  white  pines,  which  find  their  natural  home  in 


TREES,  SHRUBS,  AND  FORESTS 


381 


oaks,  poplars,  hickories,  ash,  willows,  and  other  well-known 
species  of  broad-leaved  trees.  This  hardwood  forest  of  the  central 
region  differs  from  that  of  the  coniferous  forests  in  that  it  is  not 
so  continuous,  being  composed  of  smaller  local  forest  stands  or 
of  groups  of  trees  on  farms,  known  as  the  farm  wood  lot. 

Reproduction.  The  hard  woods  belong  to  the  angiosperms,  or 
true  flowering  plants,  and  are  hence  sharply  distinguished  from 
the  cone-bearing  gymnosperms. 
The  flowers  of  the  fruit  bearers, 
such  as  the  apples  and  plums, 
have  already  been  discussed  and 
are  familiar  features  of  these 
trees  in  the  spring  on  account  of 
their  great  beauty  and  fragrance. 
Many  of  the  shade  and  timber 
trees,  however,  reproduce  by 
means  of  very  simple  flowers 
which  are  rarely  known  to  any- 
one except  the  student  of  botany. 
In  some  cases,  as  in  the  oaks 
(Fig.  238),  these  simple  flowers 
are  thought  by  many  botanists 
to  indicate  a  very  early  ancestry, 
even  antedating  that  of  herba- 
ceous species,  while  others  regard 
the  simplicity  of  the  flowers  as 
indicative  of  a  reduced  condition.  In  most  of  these  cases  polli- 
nation is  anemophilous  (by  the  wind)  and  the  trees  are  either 
monoecious  or  dioecious. 

Many  trees  produce  winged  fruits,  which  greatly  facilitate 
their  dissemination,  as  in  the  poplars,  which  so  frequently  re- 
forest burned-over  areas  on  mountain  slopes.  Other  species,  like 
the  oaks  and  hickories,  produce  heavier  nut  fruits,  which  are 
not  easily  distributed  and  hence  limit  the  range  of  these  species. 

Commercial  importance.  The  commercial  importance  of  the 
hardwood  trees  is  determined  by  the  character  of  the  fruit  and 
the  wood.  In  the  wild  state  the  wood  is  the  most  important 


FIG.  234.    Transverse  section  of  the 
wood  of  sassafras,  showing  its  ring- 
porous  character 

Photomicrograph  by  R.  B.  Hough 


382  GENERAL  BOTANY 

commercial  product  of  these  trees,  and  there  is  much  greater 
structural  variation  in  the  hard  woods  than  in  the  soft-wooded 
gymnosperms  on  account  of  the  ducts  and  fibers  in  hard  wood. 
The  hard  woods  are  all  characterized  by  the  possession  of  large 
water  ducts,  which  render  the  wood  more  porous  and  make  it 
less  uniform  in  texture  than  is  the  case  in  spruce  and  pine. 
Between  the  pores,  as  we  have  already  learned,  the  wood  is  com- 
posed of  strong  strengthening  fibers  and  living  cells,  which  are 


FIG.  235.   Diffuse-porous  woods  of  the  sycamore  and  Lolly 
Photomicrograph  by  R.  B.  Hough 

usually  more  abundant  in  the  summer  wood  than  in  the  spring 
wood  (Figs.  47  and  55).  The  large  number  of  species  of  hardwood 
trees  and  the  great  variety  in  the  character  of  the  wood  make 
this  group  of  importance  in  supplying  lumber  and  timbers  for 
almost  every  commercial  purpose.  Two  varieties  of  hard  wood  are 
recognized  in  the  industries,  namely,  ring-porous  wood  and  diffuse- 
porous  wood.  Ring-porous  wood,  like  that  of  the  sassafras 
(Fig.  234),  has  the  spring  ducts  in  the  early  spring  wood,  while 
in  diffuse-porous  woods  (Fig.  235)  the  ducts,  or  pores,  are  scat- 
tered throughout  the  entire  wood  ring.  In  ring-porous  woods 
the  annual  rings  are  more  distinct  than  in  diffuse-porous  woods. 


TREES,  SHRUBS,  AND  FORESTS 


383 


THE  WHITE  OAK  (QUERCUS  ALBA) 

Habitat  and  habit.  The  white  oak  may  be  taken  as  a  typical 
example  of  a  hardwood  forest  tree  as  compared  with  the  soft- 
wood coniferous  trees  represented  by  the  spruces  and  pines.  It 


FIG.  236.    Spreading  habit  of  the  white  oak  (Quercus  alba) 
Photograph  furnished  by  the  United  States  Forest  Service 

forms  an  important  constituent  of  the  great  central  hardwood 
forest,  where  it  attains  its  best  development  <c  on  the  western 
slopes  of  the  Allegheny  mountains  and  in  the  central  Mississippi 
and  lower  Ohio  basins."  While  the  white  oak  can  maintain  itself 
on  almost  any  soil,  the  above  habitat  indicates  its  preference  for 
the  rich,  loamy  soils  on  gentle  slopes,  bottom  lands,  and  coves. 


384 


GENERAL  BOTANY 


It  rarely  occurs  alone,  but  rather  in  mixed  forests  with  other 
oaks,  chestnuts,  basswoods,  and  tulip  trees.  The  white  oak  is 
comparatively  tolerant  in  early  life,  growing  under  the  shade  of 
other  trees  if  the  canopy  is  not  too  low  or  too  dense.  In  such 

situations  it  grows 
very  slowly  for  a 
long  period  (as  many 
as  fifty  years),  when 
it  will  take  on  new 
life  and  grow  with 
great  rapidity  if  the 
trees  which  shade  it 
are  logged  so  as  to 
expose  it  to  the  sun- 
light. When  full- 
grown  it  is  of  the 
spreading  type,  and 
its  top  forms  a  broad 
dome  with  numerous 
branches  (Fig.  236). 
It  usually  reaches  a 
height  of  from  60  to 
100  feet,  with  a  diam- 
eter of  from  2  to  4 
feet,  although  it  may 
grow  to  a  height  of 
150  feet,  with  a  di- 
ameter of  from  6  to 
8  feet.  The  bark  on 
the  young  twigs  and 
branches  is  greenish  gray,  but  on  the  mature  trunk  it  varies 
from  pale  gray  to  white,  with  shallow  fissures  and  flaky  scales. 
The  leaves  are  deeply  lobed,  with  the  rounded  lobes  charac- 
teristic of  the  white,  as  distinct  from  the  black,  oaks.  The  buds 
in  the  wintering  shoots  are  short  and  blunt,  reddish  gray  in 
color,  and  sometimes  covered  with  a  distinct  bloom.  The  white 
oak  is  easily  identified  by  the  above  characteristics. 


FIG.  237.    Stand  of  young  white  oak  timber  on 

bottom  land  of  a  small  creek 
Photograph  furnished  by  United  States  Forest  Service 


TREES,  SHRUBS,  AND  FORESTS 


385 


Reproduction.  The  white  oak  is  monoecious,  with  the  imperfect 
flowers  borne  on  the  young  shoots  and  the  twigs  of  the  previous 
season  (Fig.  238).  The  male  flowers  occur  in  loose  spikes  or 
catkins,  and  the  female  flowers  in  few-flowered  spikes  in  the  axils 
of  leaves  above  the  clusters  of  male  catkins.  Each  male  flower 
is  composed  of  a  group  of  from  5  to  8  stamens  in  an  inconspicuous 
perianth  with  from  5  to  9 
lobes.  Each  female  flower  is 
composed  of  a  single  pistil 
with  a  rudimentary  perianth. 
The  two  or  three  pistillate 
flowers  which  occur  in  a  single 
spike  are  enveloped  in  a  cup 
formed  of  coherent  bracts, 
which  ultimately  develop  into 
the  cupule,  or  cup,  of  the  fruit. 

Pollination  is  by  the  wind 
(anemophilous).  For  this  the 
white  oak  is  adapted  by  its 
abundant  light  pollen  grains 
produced  in  pendent  catkins, 
which  are  easily  swayed  by 
air  currents. 

The  fruit  is  a  nut,  or  acorn, 
resting  in  the  cupule,  or  cup, 
formed,  as  indicated  above, 
by  bracts  which  surround  each 
spike  of  female  flowers.  In 
the  white  oak  the  acorn  matures  each  fall,  unlike  the  black-oak 
series,  in  which  the  acorn  ripens  the  second  season  after  pollina- 
tion. The  acorn  germinates  in  the  fall,  sending  a  long  taproot 
deep  down  into  the  soil.  The  plumule  and  root  are  liberated 
from  the  seed  by  the  lengthening  of  the  petioles.  The  age  at 
which  the  white  oak  forms  its  seeds  varies  with  the  conditions 
•under  which  it  is  growing.  Where  trees  grow  in  the  open  in 
rich,  well-watered  soil,  seed  production  may  occur  in  the  eighteenth 
or  twentieth  year;  in  open  woods  it  occurs  at  the  age  of  forty 


Wvule. 

Female  flower 
in  long  section  Acorn 

FIG.  238.  Drawing  of  the  inflorescence, 
flowers,  and  fruit  of  the  white  oak 


386  GENERAL  BOTANY 

years ;  but  "  under  normal  forest  conditions  acorns  are  not 
produced,  as  a  rule,  before  the  seventieth  or  eightieth  year." 
Moreover,  a  large  proportion  of  the  seed  is  destroyed  by  worms, 
rodents,  and  other  animals  having  access  to  the  forests  or  wood 
lots  where  oak  trees  are  found.  The  heavy  acorns  are  distrib- 
uted mainly  by  gravity  or  by  squirrels  which  bury  them  and 
never  dig  them  up  again.  The  white  oak,  therefore,  is  not  so 
well  adapted  as  trees  with  winged  seeds,  like  the  spruces  and 
pines,  to  disseminate  itself  where  it  has  been  destroyed  by  lum- 
bering or  by  forest  fires.  Vegetative  reproduction  often  takes 
place,  however,  in  such  situations  by  the  outgrowth  of  sprouts 
from  the  stumps,  which  maintain  this  power  of  vegetative 
reproduction  for  long  periods. 

Commercial  importance.  White  oaks  are  chiefly  valuable  for 
timber  and  lumber,  although,  where  they  are  native,  they  form  a 
beautiful  ornamental  tree  with  dense  shade.  The  wood  (Figs.  47 
and  55)  is  very  dense  and  hard,  with  great  resistance  to  disease 
and  decay,  both  when  alive  and  when  used  for  railroad  ties  or 
timber  in  construction  work.  Its  main  uses  are  for  sawed  lum- 
ber, switch  ties,  and  timbers  for  ships  and  bridges  and  for  tight 
cooperage. 

A  considerable  portion  of  the  lumber  from  the  best  logs  is 
quarter-sawed  and  then  shipped  to  furniture  and  cabinet  factories, 
where  it  is  used  as  a  substitute  for  other  woods,  such  as  black 
walnut,  cherry,  and  mahogany.  The  broad  wood  rays  add  greatly 
to  the  beauty  and  value  of  quarter-sawed  oak  (Fig.  50).  Tight- 
cooperage  staves  were  early  made  from  white  oak  for  the  preser- 
vation and  shipping  of  alcohol,  wines,  and  liquors,  as  well  as  for 
oil,  molasses,  and  other  fluid  or  semifluid  substances  which  must 
be  confined  in  tight  receptacles.  The  compact,  hard  wood  of  the 
white  oak  is  admirably  fitted  for  such  purposes.  In  shipbuilding 
and  in  making  piles  for  bridges  white  oak  is  also  very  valuable, 
for  the  reason  that  it  does  not  readily  deteriorate  in  situations 
where  it  is  subjected  to  alternate  dry  and  moist  conditions. 

Like  most  of  the  valuable  forest  trees  white  oak  is  rapidly, 
diminishing  in  amount  and  quality,  so  that  its  propagation  is 
an  important  forestry  problem. 


TREES,  SHRUBS,  AND  FORESTS 


THE  WILLOWS  (SALICACEAE) 


387 


Habitat  and  habit.  Most  of  the  willows  are  hydrophytic  in 
their  habitat,  growing  along  the  borders  of  lakes  and  streams, 
in  swamps,  or  in  moist  soils  (Fig.  239).  A  few  species  are 
adapted  to  mesophytic  conditions  and  are  capable  of  cultivation 
in  comparatively  dry  soils.  In  habit  the  majority  are  shrubs, 


FIG.  239.   Natural  habitat  and  habit  of  willows  along  a  creek 
Photograph  furnished  by  the  United  States  Forest  Service 

while  a  few  forms  grow  to  be  large-sized,  spreading  trees  from 
50  to  100  feet  in  height,  attaining  to  a  diameter  of  two  or 
three  feet. 

The  bark  of  the  young  twigs  (Fig.  240)  is  smooth  or  hairy, 
while  that  on  the  main  trunk  and  the  older  branches  is  flaky. 
The  leaves  are  long  and  narrow,  with  an  entire  or  slightly 
notched  margin  and  with  a  short  petiole  bearing  stipules,  or 
appendages,  at  its  base,  where  it  joins  the  main  stem. 


388 


GENERAL  BOTANY 


Reproduction.  The  flowers  of  the  willows  occur  in  the  form  of 
catkins  (Fig.  240),  which  are  a  modified  form  of  a  spike  in  which 
the  bracts  are  represented  by  conspicuous  scales.  The  catkins  of 
the  pussy  willow  (Salix  discolor)  are  thick  and  oval  and  have 
received  the  name  pussy  on  account  of  the  silky  hairs  that  clothe . 

the  young  catkins 
in  early'  spring. 
Since  these  cat- 
kins are  borne 
on  separate  trees, 
the  willows  are 
dioecious  and  are 
entomophilous,  or 
cross-pollinated  by 
the  insects  which 
visit  the  flowers 
for  the  nectar 
secreted  by  nec- 
tar glands  at  the 

Female  flower         base   of   the  flow- 
Fewle "catkin  ^      The     ^^ 

is  -abundant  and 
sticky,  so  that 
frequent  pollina- 
tion is  assured 
although  the  flow- 
ers occur  early 
in  the  spring. 

The  individual  flowers  are  very  simple,  the  females  consisting  of 
a  simple  compound  ovary  composed  of  two  sporophylls  subtended 
by  a  hairy  bract.  The  male  flowers  bear  two  or  more  stamens, 
according  to  the  species,  in  the  axil  of  a  hairy  bract  (Fig.  240). 
The  fruit  is  a  capsule,  or  pod,  and  the  seeds  ripen  early  in  the 
spring,  after  fertilization.  The  two  sporophylls  comprising  the 
fruit  separate  so  as  to  liberate  the  seeds  when  they  are  ripe. 
The  long,  silky  hairs  of  the  seeds  facilitate  their  distribution 
by  the  wind,  or  by  water  currents. 


Seed 


Female  catkin 
(fruiting  stage) 


Fruit 


Fruit  opening 
(dehiscence) 


FIG.  240.   Inflorescence,  flowers,  and  fruit  of  willow 


TEEES,  SHEUBS,  AND  FOEESTS 


389 


The  poplars,  which  are  closely  related  to  the  willows,  have  a 
similar  natural  habitat,  but  they  grow  to  a  greater  size  and  are 
of  more  value  for  timber  or  as  ornamental  trees  than  the  willows. 
The  reproductive  organs  of  willows  and  poplars  are  similar  in 
that  the  simple  flowers  of  the  poplars  are  borne  in  catkins,  as  in 
the  willows  (Fig.  241).  The  individual  flowers  of  the  poplar, 
however,  differ  in  detail  from  those  of  the  willow.  The  stamens 
in  the  poplar  are  borne  in  considerable  numbers  on  a  bract  with 


Leaf 


Catkin 
'(female) 


Stem  section 


Catkin, 
(male} 


Male  flower 


Fruit 


FIG.  241.  Vegetative  and  reproductive  parts  of  a  poplar 

a  hairy  or  roughened  margin.  The  pistil  resembles  more  nearly 
that  of  the  willow,  but  is  characterized  by  a  larger  and  much- 
lobed  stigmatic  surface.  Each  pistillate  flower  is  in  the  axil  of 
a  bract  like  that  which  bears  the  stamens.  The  seeds  in  the  cofc 
tonwood  (PopuluB  deltoides)  are  well  known  for  their  cottony 
covering  of  long,  silky  hairs.  These  hairs  are  outgrowths  of 
the  seed  coat,  and  when  the  fruit  opens  they  expand  and  the 
seeds  can  then  be  blown  about  by  air  currents.  Pollination  in 
the  poplars,  unlike  the  willows,  is  effected  by  the  wind  (that  is, 
they  are  anemophilous),  and  the  pollen,  as  in  all  wind-pollinated 
plants,  is  produced  in  great  profusion  by  the  conspicuous  pendent 
male  catkins,  from  which  the  pollen  is  easily  scattered. 


390 


GENERAL  BOTANY 


ACERACEAE    (MAPLE   FAMILY) 
THE  SUGAR  MAPLE  (ACER  SACCHARUM) 

The  maple  family  includes  about  seventy  species,  among  which 
are  the  common  soft,  or  silver,  maple,  the  ash-leaved  maples, 
or  box  elders,  and  the  various  kinds  of  hard  maples,  of  which 

the  sugar  maple 
( Acer  saccharum) 
has  been  chosen 
for  the  following 
sketch. 

Habitat.  Rock, 
or  sugar,  maples 
generally  flourish 
best  in  a  fertile, 
loamy  soil,  but 
can  adapt  them- 
selves to  other 
kinds  of  soils  if 
they  have  a  me- 
dium amount  of 
moisture.  This 
tree  is  distinctive 
hi  habit  because 
of  its  symmetri- 
cal, broad,  round 
top,  so  character- 
istic of  old  trees. 

Male  flowers  Female  flowers  ^  j^  Qn  Qlder 

FIG.  242.   Flowers,  fruits,  and  seeds  of  the  sugar  maple 
(Acer  saccharum) 


Leaf. 


Leaves,  buds,  and  fruits 


Terminal  twig 


Female  flowers 


Inflorescence  fascicle 


Samara,  or 
key'fruil 


trees     is     deeply 
furrowed   and    is 

divided  into  broad  plates  or  large  flakes.    The  leaves  are  broad, 

thin,  and  deep  green,  with  the  margin  divided  into  five  points, 

or  lobes  (Fig.  242). 

Reproduction.   The  flowers  in  the  sugar  maple  are  borne  in 

corymblike  clusters  with  long,  hairlike  pedicels.    Some  of  the 


TREES,  SHRUBS,  AND  FORESTS 


391 


flowers  are  perfect,  while  others  are  imperfect.  The  trees  are 
therefore  either  dioecious,  with  male  flowers  on  one  tree  and 
female  flowers  on  another,  or  polygamo-moncscious,  that  is,  with 
some  imperfect  flowers  of  both  sexes  on  the  same  tree. 

The  individual  flowers  (Fig.  242)  have  a  five-lobed  perianth 
inclosing  both  sets  of  essential  organs  in  the  perfect  flowers  or 
one  set  in  the  imperfect  flowers.  The  pistillate  flowers  usually 


FIG.  243.    A  grove  of  sugar  maples  which  have  been  tapped  for 
over  eighty  years 

From  Bergen  and  CaldwelPs  "  Introduction  to  Botany  " 

contain  rudimentary  stamens.  Pollination  is  entomophilous,  and 
the  fruits  are  of  the  samara  type,  the  wings  being  formed  by  an 
outgrowth  from  the  ovary  wall. 

Distribution  and  importance.  Sugar  maple  is  distributed  over 
the  entire  eastern  part  of  the  United  States,  occurring  in  the 
northern  and  central  hardwood  forests  and  as  a  shade  and  orna- 
mental tree.  The  wood  is  diffuse  porous,  with  the  ducts 
scattered  throughout  the  annual  rings  and  separated  by  dense 
masses  of  wood  fibers.  It  is  preeminently  a  manufacturer's  wood, 
the  demand  being  that  of  a  planing-mill  product.  These  products 


392 


GENERAL  BOTANY 


include  flooring,  ceiling,  and  molding  and  door  lumber.  In 
addition  the  wood  is  used  for  a  great  variety  of  purposes,  —  for 
agricultural  implements,  musical  instruments,  tools,  and  excel- 
sior. Curly  and  bird's-eye  maple  (Fig.  49)  are  of  special  value 
for  interior  finish  and  decorative  effects.  The  curly  effect  is  due 


FIG.  244.   Habit  of  the  American  elm  (Ulmus  americana)  in  summer 
Photograph  furnished  by  the  United  States  Forest  Service 

to  the  fact  that  the  grain  is  wavy,  instead  of  straight  as  in  most 
trees.  Bird's-eye  effects  may  be  due  to  the  distortion  of  the  annual 
wood  rings  by  latent  buds,  unsuccessful  branches,  or  injuries  pro- 
duced by  wood  borers.  The  maple-sugar  industry  is  also  one  of 
considerable  importance  in  the  east  and  in  the  states  of  Michigan, 
Wisconsin,  and  New  York.  For  this  purpose  the  sap  of  the  black 
maple  and  that  of  the  sugar  maple  are  the  most  valuable. 


TEEES,  SHEUBS,  AND  FOEESTS 


393 


ULMAUEAE  (ELM  FAMILY) 
THE  WHITE  ELM  (ULMUS  AMERICANA) 

Habitat  and  habit.  The  white  elm,  with  its  near  relatives  the 
slippery  elm  (Ulmus  fulva)  and  the  cork  elm  (Ulmus  racemosa*), 
occupies  a  wide  area  in  the  United  States  from  the  Rocky 
Mountains  to  the 
eastern  seaboard.  In 
these  regions  it  does 
not  form  pure  forest 
stands  like  many  other 
forest  trees,  but  occurs 
with  such  moisture- 
loving  trees  as  the 
cottonwoods,  silver 
maples,  and  syca- 
mores along  streams, 
in  river  bottoms,  in 
farm  wood-lots,  and 
on  lawns  where  there 
is  good,  well-drained 
soil.  It  rarely  occurs 
among  oaks,  pines, 
and  hickories,  which 
grow  in  drier  soils. 

The  elm  should 
also  be  classed  as  an 
intolerant  tree,  since 
it  needs  plenty  of 
light  for  its  best  de- 
velopment. The  graceful  habit  of  the  elm  makes  it  one  of  the 
most  valuable  of  trees  for  ornamental  purposes  on  private  lawns 
and  as  a  shade  tree  in  public  parks  and  city  streets. 

Reproduction.  The  flowers  of  the  elms  occur  in  dense,  raceme- 
like  clusters,  or  fascicles,  which  open  in  early  spring  before  the 
leaves  are  out  (Fig.  247).  The  flowers  on  different  parts  of 


FIG.  245.   Tree  trunk  and  bark  of  the  white  elm 
Photograph  furnished  by  United  States  Forest  Service 


394 


GENERAL  BOTANY 


the  tree  are  of  various  sorts,  some  being  perfect,  with  both  pistils 
and  stamens  in  the  same  flower,  while  others  are  imperfect,  with 
either  stamens  or  pistils  in  a  single  flower.  In  the  latter  instances 
both  sets  of  organs  are  usually  present,  but  one  set  (either  pis- 
tils or  stamens)  is  abortive  and  hence  useless  in  reproduction. 
These  abortive  organs  furnish  an  interesting  example  of  the  per- 
sistence of  structures  which  were  once  useful  but  have  ceased  to 
function.  Such  a  floral  condition,  with  a  mixture  of  perfect  and 
imperfect  flowers  on  the  same  tree,  is  designated  as  polygamous. 


FIG.  246.   Natural  habitat  of  elms  on  a  creek  bottom 
Photograph  furnished  by  the  United  States  Forest  Service 

Pollination  is  necessarily  either  self-pollination  or  cross- 
pollination  and  is  effected  by  wind  or  by  insects.  The  fruit  is  a 
winged  samara,  or  key  fruit,  the  wings  being  an  outgrowth  of 
the  ovary  wall  which  takes  place  as  a  secondary  result  of  fertili- 
zation along  with  the  formation  of  the  embryo  and  seed.  This 
winged  fruit  aids  materially  in  the  dissemination  of  the  elms. 

Commercial  importance.  In  addition  to  its  value  as  a  shade 
and  ornamental  tree  the  elm  is  of  considerable  importance  in  the 
production  of  lumber  for  certain  purposes.  The  wood  is  of  the 
ring-porous  type,  with  a  single  circle  of  large  spring  ducts  in 


TEEES,  SHEUBS,  AND  FOEESTS 


395 


each  annual  ring.  The  remainder  of  the  ring  is  composed  of 
alternating  bands  of  small  ducts  and  bands  of  fibers.  The  wood 
rays  and  wood  parenchyma  are  inconspicuous.  The  value  of 
elm  for  commercial  purposes  is  due  to  its  strength  and  to  the 
qualities  which  enable  it  to  withstand  bending  and  shock  better 
than  many  other 
kinds  of  woods. 
Consequently  its 
greatest  use  is  in 
industries  such  as 
slack  cooperage 
and  the  making 
of  baskets,  boxes, 
crates,  and  vehi- 
cles. In  the  mak- 
ing of  hoops  and 
staves  for  coop- 
erage, as  well  as 
of  rims  or  bands 
in  baskets  and 
crates,  its  bend- 
ing quality  and 
its  strength  are  of 
particular  value. 
In  vehicles  it  is 
used  chiefly  in 
body  work.  Elm 

wood  is  also  used  for  a  great  variety  of  industries,  such  as  the 
making  of  chairs,  musical  instruments,  agricultural  tools,  etc. 
The  principal  sources  of  elm  wood  are  the  states  bordering  on 
the  Great  Lakes  (particularly  Michigan  and  Wisconsin)  and  those 
in  the  southern  Mississippi  region  (that  is,  Arkansas,  Mississippi, 
and  Louisiana).  Like  most  valuable  woods,  however,  it  is  being 
rapidly  depleted  and  must  either  be  replanted  or  replaced  by 
other  woods  of  a  similar  character. 


FIG.  247.   Vegetative  and  reproductive  structures  of  the 
red,  or  slippery,  elm  ( Ulmus  fulva) 

a,  winter  twig ;  b,  leaf ;  c,  fascicle  of  flowers ;  d,  single  per- 
fect flower;    e,   winged  fruit.     From   "Michigan   Trees." 
Drawing  furnished  by  Dr.  Charles  W.  Otis 


CHAPTER  XX 

HERBACEOUS  AND  WOODY  DICOTYLEDONS 

The  dicotyledons  are  the  largest  and  probably  the  oldest 
group  of  true  flowering  plants ;  the  monocotyledons  are  supposed 
to  have  originated  from  them  in  late  geologic  times.  The  dicoty- 
ledons comprise  over  a  hundred  thousand  known  species,  ranging 
from  trees  and  shrubs  to  the  simplest  types  of  herbaceous  plants. 
They  are  also  adapted  to  the  most  diverse  conditions  of  soil  and 
climate,  being  widely  disseminated  o^er  all  parts  of  the  earth  as 
mesophytes,  hydrophytes,  and  xerophytes.  Their  chief  distinctive 
characteristics  are  their  broad,  net-veined  leaves,  the  active  cam- 
bium of  the  stem,  the  floral  plan,  and  the  two  cotyledons  of  the 
embryo  which  give  to  the  group  the  name  dicotyledons. 

The  following  representative  families  and  species  are  discussed 
in  order  to  furnish  an  introduction  to  the  biological  and  economic 
aspects  of  the  group. 


RANUNCULACEAE  (BUTTERCUP  FAMILY) 

The  Ranunculaceae  include  many  of  our  commonest  and  most 
typical  plants  of  the  spring  flora.  Among  these  the  buttercups,  the 
marsh  marigold,  the  hepaticas,  and  the  wood  anemones  are  early 
harbingers  of  spring,  which  represent  at  the  same  time  the  simpler 
types  of  plants  in  the  family,  with  regular  spiral  and  hypogynous 
flowers.  Among  the  more  highly  modified  and  showy  flowers  of 
the  family  the  columbines,  larkspurs,  and  clematis  represent  both 
wild  and  cultivated  species  of  great  beauty  and  usefulness  as 
ornamental  plants.  Most  of  the  species  are  stemless  herbs  with 
simple  leaves,  which  produce  their  flowers  and  fruits  early 
in  the  spring. 

396 


HERBACEOUS  AND  WOODY  DICOTYLEDONS      397 


CALTI1A   (MARSH  MARIGOLD)  AND  RANUNCULUS  (BUTTERCUP) 

Habitat  and  habit.  Caltha  palustris,  the  marsh  marigold,  differs 
from  the  majority  of  the  buttercups  in  being  a  hydrophyte, 
inhabiting  marshes  and  swamps  along  the  margins  of  streams 
and  lakes.  The  heart-shaped,  rounded  leaves  are  mostly  radi- 
cal, or  root,  leaves  (Fig.  248),  but  some  leaves  also  are  found 
on  the  flowering 
stems,  which  grow 
up  annually  and 
produce  clusters 
of  bright  yellow 
flowers.  The  but- 
tercups are  quite 
diversified  in  both 
habitat  and  habit, 
being  represented 
by  true  mesophytes 
and  hydrophytes 
as  well  as  by  cau- 
lescent and  acau- 
lescent  species. 

Reproduction.  In 


FIG.  248.   Habit  and  reproductive  organs  of  the  marsh 
marigold  (Caltha  palustris) 


a,  the  plant;  6,  stamen;  c,  long  section  of  a  pistil,  showing 
the  ovules  on  the  placenta ;  d,  ovule ;  e,  floral  diagram 


both  Caltha  and 
the  true  butter- 
cups the  flowers 
are  simple  and 
regular,  and  are  thus  representative  of  the  simplest  types  of 
floral  structure  among  the  plants  of  the  Ranunculus  family. 
The  account  already  given  of  the  flower  of  Caltha  indicates  its 
close  alliance  with  the  spiral,  hypogynous  flowers  whose  struc- 
ture is  similar  to  that  of  a  strobilus  (Fig.  200). 

Cross-pollination  is  provided  for  in  both  Caltha  and  the  butter- 
cups, although  close-pollination  and  self-pollination  are  possible 
in  most  cases.  The  following  account  of  the  adaptation  of  the 
flowers  to  pollination  is  applicable  to  both  Caltha  and  the 
common  buttercups. 


398 


GENERAL  BOTANY 


In  the  young  flowers  the  stamens  are  massed  closely  around 
the  pistils,  with  the  anthers  just  below  the  stigmatic  surfaces. 
In  the  older  flowers  the  outer  stamens  ripen  first,  and  since  the 

, ,     anthers  are   extrorse 

(that  is,  on  the  outer 
abaxial  surface  of  the 
filament),  the  pollen 
as  it  is  shed  is  not 
likely  to  reach  the 
stigmas  without  in- 
sect aid.  The  dehis- 
cence  of  the  remaining 
anthers,  after  the  first, 
is  centripetal  (that  is, 
from  the  outside  of 
the  flower  toward  the 
centrally  located  pis- 
tils), which  further 
insures  against  self- 
pollination.  Insect 
visitors  are  attracted 
by  the  bright  yellow 
color  of  the  petals 
and  by  the  abundant 
nectar,  which  is  se- 
creted in  two  shallow 
depressions  on  either 
side  of  each  ovary. 
When  they  alight 
upon  the  center  of 
the  flower  with  pol- 
len on  the  abdomen  derived  from  other  flowers,  they  effect 
either  close-pollination  or  cross-pollination,  according  to  the 
source  of  the  pollen.  Insects  which  pass  from  the  outer  border 
of  a  flower  across  the  center  are  likely  to  effect  self-pollination. 
The  fruit  is  a  many-seeded  follicle  which  splits  down  the  placen- 
tal  suture  and  liberates  the  seeds. 


Courtesy  of  American  Magazine  of  Forestry,  Washington,  D.  C. 

FIG.  249.    Habit  of  the  common  buttercup 
Photograph  by  Dr.  R.  W.  Shufeldt 


HERBACEOUS  AND  WOODY  DICOTYLEDONS      399 


Anther 


AQUILEGIA  (COLUMBINE) 

In  the  columbines  the  flowers  are  very  highly  modified  and 
irregular,  the  five  petals  being  developed  into  long  spurs  with 
nectaries  at  the  tips.  The  flowers  are  pendulous,  and  the  nectar 
secreted  in  the  end  of  the  spur  is  thus  protected  from  rain.  They 
are  pollinated  by  bumblebees,  which  possess  a  proboscis  long 
enough  to  penetrate  to  the  nectar  at  the  base  of  the  spur,  which 
may  be  from  15  to 
20  mm.  in  length 
(Fig.  250,  a). 

The  following 
account  of  the 
pollination  of 
Aquilegia  vulga- 
m,  taken  from 
Knu  th's  Hand- 
book, will  serve 

to    indicate    the  c  "'^Stigmas 

details    ill   Other  FIG.  250.   Flowers  of  the  columbine  (Aquilegia) 

species.  "  Bum- 
blebees, which 
possess  a  probos- 
cis long  enough  to  reach  the  nectar  in  the  legitimate  way,  cling 
to  the  flowers  from  below,  holding  011  to  the  base  of  the  spur 
with  the  forelegs  and  pushing  the  head  into  the  spur.  In  so 
doing  the  underside  of  the  abdomen  comes  in  contact,  in  the 
young  flowers,  with  the  pollen-covered  anthers  which  surround 
the  pistil  (Fig.  250,  I).  In  older  flowers  (c)  the  same  region  of 
the  body  touches  the  somewhat  spreading  stigmas  which  project 
from  among  the  stamens,  so  that  cross-pollination  is  necessarily 
effected.  Should  insect  visits  fail,  automatic  self-pollination 
readily  takes  place,  for  the  carpels  (pistils)  grow  down  in  the 
middle  of  the  stamens,  and  by  elongation  of  the  style  the  stigmas 
ultimately  reach  a  lower  level  than  the  anthers."  Other  mem- 
bers of  the  Ranunculaceae  are  also  highly  modified  for  securing 
cross-pollination  by  insects. 


ntfier 


a,  single  flower;  6,  young  flower  with  anthers  surrounding 
the  stigmas;  c,  older  flower  with  pollen^shed  and  stigmas 
protruding.  FromKnuth's  "Handbook  of  Flower  Pollination  " 


400 


GENEKAL  BOTANY 


CRUCIFERAE  (MUSTARD  FAMILY) 

The  Oruciferae  include  some  of  the  most  important  culti- 
vated commercial  varieties  of  plants,  such  as  the  cabbage,  turnip, 
radish,  and  cress.  The  most  distinctive  features  of  the  family  are 
the  acrid  juice  and  the  peculiar  structure  of  the  flowers  and  fruit. 

Reproduction.  The  parts  of  the  flowers  are  in  sets  of  four 
members  each,  the  petals  being  so  related  as  to  form  a  cross, 
so  that  the  flowers  are  said  to  be  cruciate.  In  Fig.  251,  a,  the 
flower  of  one  of  the  turnips  (Brassica)  is  shown  in  long  section, 
so  as  to  illustrate  the  relations  of  the  floral  parts.  The  pistil 
is  composed  of  two  sporophylls,  or  carpels,  and  the  cavity  of 


Carpel 


FIG.  251.   Flowers  and  fruit  of  a  cruciferous  plant  (Brassfca) 

a,  young  flower  with   unripe  anthers;  6,  older  flower,  anthers  shedding  pollen; 
c,  floral  plan ;  d,  fruit  (silique) 

the  ovary  is  separated  by  a  false  partition,  or  septum,  which 
runs  between  the  two  placental  ridges  (see  the  section  of  the 
ovary  in  the  ground  plan  of  a  cruciferous  flower,  Fig.  251,  c). 

The  stamens  are  six  in  number,  two  short  and  four  long,  the 
four  long  stamens  representing  two  stamens  which  branch  at 
the  base.  Cross-pollination  may  be  effected  through  the  agency 
of  insects,  or  self-pollination  from  the  anthers  of  the  long  stamens. 
When  the  flowers  are  young  (Fig.  251,  a),  the  anthers  of  the  long 
stamens  are  often  separated  from  the  stigma  by  the  movements 
of  the  filaments.  In  more  mature  flowers  (£)  the  anthers  move 
toward  the  stigma,  so  that  self-pollination  is  effected.  The  fruit 
(ef)  is  a  pod,  called  a  silique.  When  ripe  the  two  halves  of  the 
pod  separate  from  the  septum,  or  partition,  to  which  the  seeds 
cling,  and  the  seeds  are  then  disseminated. 


HERBACEOUS  AND  WOODY  DICOTYLEDONS      401 

Commercial  varieties.  The  commercial  varieties  of  the  Cru- 
ciferae  are  of  the  greatest  importance  economically  and  are  also 
of  special  interest  to  the  student  of  variation  and  development 
in  cultivated  plants,  since  some  very  remarkable  transformations 
have  been  effected  in  them  by  culture.  Thus,  in  the  cabbage 
the  stem  has  become  greatly  shortened  and  the  normal  leaves 
transformed  into  the  overlapping,  fleshy  leaves  of  the  cabbage 
head.  In  Brussels  sprouts  the  stem  has  retained  its  normal  length, 


Courtesy  of  American  Magazine  of  .ForesfryTWashington,  D.C. 

FIG.  252.    Habit  of  violets  with  flowers 
Photograph  by  Dr.  R.  W.  Shufeldt 

but  the  lateral  buds  have  been  enlarged  until  they  look  like 
small  cabbage  heads.  In  turnips  and  radishes  the  main  tap- 
root has  become  highly  modified  into  the  fleshy  roots  of  the 
commercial  varieties.  In  the  transformation  of  the  cabbages, 
cauliflower,  Brussels  sprouts,  etc.  mutation  is  thought  to  have 
occurred,  thus  giving  rise  to  some  of  the  striking  variations  in 
these  plants  which  have  been  accentuated  by-  cultivation. 

VIOL  ACE  AE  (  VIOLET  FAMILY) 

The  Violaceae  are  of  special  interest  on  -account  of  their 
great  beauty,  both  in  the  wild  state  and  under  cultivation.  The 
cultivated  pansy  is  a  hybrid  of  several  wild  species  of  violets. 


402 


GENERAL  BOTANY 


Biologically  considerable  interest  attaches  to  these  beautiful 
plants  on  account  of  the  unusual  devices  which  result  in  cross- 
pollination  by  insects.  Fig.  252  illustrates  the  usual  habit  of 
the  violet  plants  and  the  highly  modified  floral  structure.  In 
Fig.  253  a  sectional  view  of  the  flower  of  Viola  pinnata  is  also 
shown,  which  will  enable  the  student  to  understand  more  fully 
the  wonderful  mechanism  for  securing  pollination  and  fertiliza- 
tion in  this  species.  The  lower  petal  of  the  flower  is  projected 

backward  in  the  form 
of  a  spur,  into  which 
two  nectaries  grow  as 
continuations  of  the 
filaments  of  the  two 
lowest  anthers.  The 
stamens  form  a  cone 
around  the  style,  which 
is  closed  except  on  the 
lower  side  between  the 
two  lowest  anthers. 
The  style  projects  be- 
yond the  stamen  cone, 
bearing  the  stigma  at 
its  end.  When  an  insect 
probes  down  the  cavity 
of  the  spur  for  nectar 
from  the  nectaries,  he  spreads  the  lower  anthers  slightly,  which 
allows  pollen,  already  dehisced  into  the  anther  cone,  to  sift  out 
upon  his  proboscis.  This  pollen  cannot  reach  the  stigmatic  sur- 
face of  the  same  flower  upon  the  withdrawal  of  the  proboscis,  on 
account  of  its  position  and  structure.  When,  however,  the  next 
flower  is  visited  for  nectar,  the  stigma  is  almost  certain  to  be 
dusted  with  foreign  pollen  gained  from  a  previously  visited  flower, 
and  so  to  be  either  close-pollinated  or  cross-pollinated.  The 
fruit  is  a  pod  which  opens  by  three  valves,  and  the  seeds  are  often 
thrown  to  considerable  distances  by  a  special  mechanism.  The 
two  walls  of  each  valve  contract  as  they  become  dry,  and  squeeze 
the  hard  seeds,  which  are  thus  thrown  from  the  mother  plant. 


FIG.  253.    Structure  of   the  flower  of  a  violet 
with  reference  to  pollination 

Adapted  from  Knuth's  "Handbook  of 
Flower  Pollination  " 


HERBACEOUS  AND  WOODY  DICOTYLEDONS      403 


Eos  ACE  AE  (Ross  FAMILY) 

The  rose  family  comprises  over  14,000  species,  including  an 
unusually  large  number  of  ornamental,  food-producing,  and  fruit- 
producing  varieties  of  great  value  and  importance  to  man.  The 
family  is  also  noted  for  the  great  variation  in  its  species  as  re- 
gards habit,  since  it  includes  all  types  of  plants  from  acaulescent 
herbs,  like  the  potentil- 
las  and  strawberries,  to 
shrubs  and  trees,  like 
the  roses  proper,  the 
spireas,  and  the  apples, 
pears,  and  plums. 

In  the  treatment  of 
the  following  represent- 
ative species  emphasis 
will  be  placed  largely 
upon  reproduction  and 
fruit  formation,  since 
this  is  the  distinctive 
feature  of  the  rose 
family  which  is  of  the 
greatest  interest  and 
importance. 


FRAG AKI A  (STRAW- 
BERRY) 


Courtesy  of  American  Magazine  of  Forestry,  Washington,  D.C. 

FIG.  254.    Habit  and  flowers  of  the 
American  strawberry 

Photograph  by  Dr.  R.  W.  Shufeldt 


Habit    and   history. 

The  cultivated  straw- 
berry is  a  direct  de- 
scendant of  one  of  the 

wild  strawberries,  and  its  large  size  and  greatly  improved  fruit 
is  due  to  cultivation  and  breeding  by  man  during  the  last  fifty 
years.  Professor  L.  H.  Bailey,  a  recognized  authority  on  the 
evolution  of  our  native  fruits,  ascribes  the  origin  of  our  present 
varieties  of  cultivated  strawberries  to  the  improvement  of  a  wild 
species  from  Chile,  known  as  the  Chilean  strawberry  (Fragaria 


404  GENEKAL  BOTANY 

chiloensis).  Like  most  of  the  other  fruits  of  this  order  the  straw- 
berry is  therefore  a  product  of  the  breeding  and  improvement  of 
wild  plants  by  the  methods  of  hybridizing  and  breeding  recorded 
in  earlier  chapters.  Vegetative  reproduction  by  means  of  run- 
ners has  already  been  explained  in  Part  I  and  is  there  illustrated, 
together  with  the  acaulescent  habit  of  these  plants,  in  Fig.  76. 
Flowers  and  fruit.  The  flowers  in  the  strawberry  are  quite 
similar  to  those  of  Caltha,  with  an  elongated  receptacle  on  which 
the  numerous  stamens  and  the  cluster  of  simple  pistils  are 
arranged  spirally  (Fig.  255,  a,  5).  The  flowers  are  usually  per- 
fect, but  some  commercial  varieties  have  only  pistillate  flowers, 
while  others  have  pistils  with  few  stamens  (Fig.  255,  e,/).  In 


FIG.  255.   Flowers  and  fruit  of  the  strawberry  (Fragaria) 

a,  6,  surface  and  sectional  views  of  a  perfect  flower;  c,  fruit;  d,  perfect  flower; 
e,  f,  imperfect  flowers  (e,  pistillate  flower ;  /,  staminate  flower) 

planting  such  varieties  care  must  be  taken  to  have  both  pistillate 
and  pollen-bearing  individuals  in  the  same  or  in  alternate  rows. 

Cross-pollination  is  effected  by  insects,  since  the  perfect  flowers 
are  protogynous,  the  stigmas  ripening  before  the  stamens.  Polli- 
nation and  fertilization  in  the  strawberry,  as  in  so  many  other 
fruit-forming  species,  results  not  only  in  the  development  of  the 
seeds  but  also  in  great  changes  in  the  receptacle,  by  which  it 
becomes  greatly  enlarged  and  fleshy  to  form  the  fruit.  The 
seeds  remain  inclosed  in  the  ovary  wall,  which  dries  and  forms 
a  true  achene. 

The  fruit  is  therefore  composed  largely  of  fleshy  receptacle, 
with  the  hard  achenes  scattered  over  its  surface  and  commonly 
designated  as  seeds  by  those  who  eat  the  ripened  fruit.  The  dis- 
semination of  the  wild  strawberry  is  undoubtedly  facilitated  in 
nature  by  animals  that  eat  the  fruit  and  eject  the  dried  achenes 
and  seeds  with  their  excrements. 


HERBACEOUS  AND  WOODY  DICOTYLEDONS      405 


RUBUS  (RASPBERRIES  AND  BLACKBERRIES) 

Habit  and  history.  The  raspberries  and  blackberries  now 
under  cultivation  had  the  same  general  origin  and  history  as 
that  of  the  cultivated  strawberry  in  that  they  have  been  derived 
in  almost  every  instance  from  wild  ancestors.  Thus,  the  com- 
mon black-cap  raspberries  seem  to  have  originated  from  chance 
seedlings,  possibly  mutations,  first  discovered  in  Ohio  in  1832 
by  Nicholas  Longworth.  The 
red  raspberries  probably  had 
a  similar  origin  from  wild 
ancestors  in  Canada  and  the 
northeastern  portions  of  the 
United  States.  The  culti- 
vated blackberries  have  evi- 
dently been  developed  from 
the  wild  species  of  our  own 
country  and  came  gradually 
into  use  by  the  early  settlers. 

Both  raspberries  and  black- 
berries are  prickly  shrubs  with 
simple  or  compound  leaves 
and  with  aerial  stems,  called 
canes,  which  produce  leaves 
and  buds  for  the  first  year 
only.  From  these  first-year 
canes  lateral  buds  develop  the  flowers  and  fruit  of  the  second 
season.  After  the  formation  of  the  fruit  the  canes  die  and  are 
replaced  in  many  species  by  sucker  shoots  which  spring  up  from 
the  roots.  In  the  black  raspberry  vegetative  reproduction  takes 
place  by  means  of  branches  which  bend  over  and  take  root,  thus 
giving  rise  to  new  plants  (Fig.  75). 

Flowers  and  fruit.  The  flowers  are  either  axillary  and  soli- 
tary or  are  borne  in  clusters.  They  resemble  the  flowers  of  the 
strawberry  in  being  perfect  and  perigynous,  with  an  enlarged 
receptacle  (Fig.  256,  6).  There  are  five  petals,  many  separate 
pistils  crowded  on  the  receptacle,  and  numerous  stamens. 


FIG.  256.    Flower  and  fruit  of  the 
blackberry  and  the  rose 

a,  6,  c,  blackberry;  d,  e,f,  rose 


406  GENERAL  BOTANY 

Close-pollination  and  cross-pollination  are  effected  by  insects, 
which  visit  the  flowers  for  the  nectar  secreted  by  a  ringlike 
nectary*  at  the  base  of  the  stamens. 

The  fruit  is  an  aggregate  of  small  stone  fruits,  or  drupes,  in 
both  the  raspberry  and  the  blackberry,  the  separate  drupes  arising 
from  the  ovary  wall,  which  forms*  in  ripening,  an  outer  fleshy 
wall  and  an  inner  stony  part  inclosing  the  seed.  In  the  black- 
berry (Fig.  256,  c)  these  stone  fruits  are  aggregated  on  a  fleshy 
receptacle  which,  as  in  the  strawberry,  forms  a  part  of  the  ripened 
aggregate  fruit.  In  the  raspberry  the  separate  stone  fruits  unite 
by  interlocking  hairs,  but  separate  easily  from  the  receptacle, 
which  does  not  in  this  instance  enter  into  the  edible  portion  of 
the  fruit,  as  in  the  blackberry  and  the  strawberry. 

ROSA  (ROSES) 

Habit.  The  roses  proper  are  woody  perennials,  often  becoming 
shrubby  in  habit,  with  compound  leaves  and  conspicuous  flowers 
borne  singly  or  in  clusters.  The  plants  are  principally  noted  for 
their  ornamental  effects,  due  to  the  great  size  and  beauty  of  the 
flowers  under  cultivation. 

Flowers  and  fruit.  The  flowers  of  the  wild  and  cultivated 
roses  are  perigynous,  with  an  urn-shaped  receptacle  which  bears 
the  conspicuous  petals,  calyx  tubes,  and  stamens  on  the  rim 
of  the  cup  (Fig.  256,  e).  The  pistils  are  numerous,  springing 
from  the  base  of  the  receptacle  and  inclosed  by  it. 

Pollination.  The  stigmas  protrude  beyond  the  urn-shaped  re- 
ceptacle in  the  open  flower,  and  the  stamens  bend  outward,  facil- 
itating cross-pollination,  although  self-pollination  is  not  precluded. 
In  the  development  of  the  fruit,  or  rose  hip  (Fig.  256,  /),  both 
the  pistils  and  the  receptacle  have  a  part.  The  pistils  develop 
into  hard,  bony,  achenelike  fruits  inclosed  by  the  receptacle, 
which  becomes  fleshy  and  bright  red  or  scarlet  at  maturity.  The 
stamens  and  corolla  disappear  during  fruit  formation,  but  the 
calyx  lobes  adorn  the  upper  border  of  the  rose  hip  after  it  is  ripe. 
The  seeds  are  scattered  when  the  fruit  decays,  or  they  may  be 
disseminated  by  animals  if  the  fruit  is  eaten. 


HERBACEOUS  AND  WOODY  DICOTYLEDONS      407 


PRUNUS  (CHERRY) 

Reproduction.  The  flowers  of  the  cherry  are  borne  in  lateral 
buds  on  dwarfed  shoots  called  spur  shoots.  The  flowers  are  perig- 
ynous  (Fig.  257,  /,  and  259,  &),  as  in  the  genus  Rosa,  but  there 
is  only  one  ovary  in  the  urn-shaped  receptacle  of  each  flower. 


FIG.  257.   Black  cherry  (Prunus  serotina) 

a,  winter  twig;  6,  portion  of  the  same  enlarged;  c,  d,  a  leaf  and  the  leaf  margin; 

e,  racemose  inflorescence ;  /,  perigynous  flower  in  section ;  g,  fruit  cluster.    From 

"Michigan  Trees."    Photograph  furnished  by  Dr.  Charles  H.  Otis 

In  the  ripening  of  the  fruit  the  pistil  forms  the  entire  fruit,  the 
inner  portion  of  the  ovary  wall  forming  the  stone  and  the  outer 
portion  the  fleshy  part  of  the  fruit.  The  receptacle  and  the 
floral  parts  formed  upon  its  upper  margin  disappear  with  the 
development  of  the  fruit  (Fig.  257,  g,  and  259,  c). 


408 


GENEKAL  BOTANY 


MALUS  (APPLES  AND  CKABS) 

The  cultivated  apples  and  crabs  (Fig.  258)  are  known  to  have 
had  a  common  origin  from  wild  ancestors  resembling  the  wild 
crabs  of  to-day.  In  many  instances  these  wild  ancestors  are  of 


FIG.  258.   American  crab  (Pyrus  coronaria) 

a,  winter  twigs;  6,  portion  of  twig  enlarged  showing  leaf  scar  and  bud;  d,  inflores- 
cence ;  e,  flower  in  long  section,  with  petals  removed,  showing  its  epigynous  character ; 
/,  fruit  (pome).   Photograph  furnished  by  Dr.  Charles  H.  Otis 

American  origin,  but  in  other  instances,  particularly  in  the  case 
of  apples,  the  early  ancestors  are  of  European  origin,  and  the 
cultivated  varieties  therefore  originated  on  foreign  soil.  The 
plants  are  in  all  instances  trees  of  a  diffuse  habit  of  growth  and 
are  of  no  particular  value  except  for  the  production  of  fruit. 


HERBACEOUS  AND  WOODY  DICOTYLEDONS      409 


Flowers  and  fruit.  The  flowers  in  most  common  apples  are 
borne  in  clusters,  on  dwarf  or  spur  shoots,  which  occur  as  lateral 
shoots  on  the  more  rapidly  growing  leafy  branches  of  the  tree. 
The  buds  which  bear  the  flowers  are  mixed  buds,  bearing  ter- 
minal flower  clusters  and  a  few  leaves  at  the  base  of  the  year's 
growth.  Since  the  spur  shoot 
is  usually  terminated  by  a 
flower  cluster,  its  growth  in 
length  from  this  bud  is  stopped, 
but  may  be  continued  by  a  lat- 
eral bud,  which  may  in  turn 
produce  a  terminal  mixed  bud 
and  bear  flowers  and  fruit.  The 
flowers,  unlike  those  of  the 
rose  and  cherry,  are  epigynous 
(Figs.  258,  d  and  e,  and  259,  e), 
so  that  the  receptacle  and  ovary 
are  closely  adherent  in  the  for- 
mation of  the  fruit. 

Cross-pollination  takes  place 
normally  by  insects,  but  some 
varieties  are  self -sterile ;  that 
is,  they  fail  to  be  fertilized 
when  pollinated  with  their  own 
pollen.  In  such  instances  the 
self-sterile  variety,  in  order  to 
set  its  fruit,  must  be  crossed 
with  pollen  from  some  other 
variety  with  which  it  is  fertile. 
In  the  development  of  the  fruit  the  ovary  wall  forms  the  mem- 
branous core  and  a  portion  of  the  flesh  immediately  surrounding 
it.  The  outer  and  greater  portion  of  the  fleshy  part  of  the  apple 
is,  however,  developed  from  the  receptacle. 

In  the  apple  and  many  other  members  of  the  rose  family  the 
seeds  are  disseminated  mainly  by  animals,  especially  birds,  which 
eat  the  fruits  and  the  seeds.  The  hard  seeds  resist  the  action 
of  the  digestive  juices  and  are  cast  out  with  animal  excrements, 


FIG.  259.    Flower  and  fruit  of  the 
cherry  and  the  apple 

b,  perigynous  flower  of  the  cherry,  in 
section;  c,  section  of  the  drupe;  e,  epig- 
ynous flower  of  the  apple,  in  section; 
f,  section  of  the  pome 


410 


GENERAL  BOTANY 


often  at  great  distances  from  the  mother  plant.  Marked  excep- 
tions to  the 'above  rule  occur  in  G-eum  triflorum,  which  has  hairy 
pistils  which  elongate  and  distribute  the  seeds  by  the  wind. 


COMPOSITAE  (COMPOSITE  FAMILY) 

The  Compositae  are  the  highest  type  of  the  Sympetalae,  or 
flowers  with  the  petals  of  the  corolla  united  instead  of  separate 
as  in  the  other  plants  thus  far  studied.  They  are  represented  by 


Courtesy  of  American  Magazine  of  Forestry,  Washington,  D.C. 

FIG.  260.   Habit  and  flowers  of  the  dandelion  (Taraxacum) 
Photograph  hy  Dr.  R.  W.  Shufeldt 

such  common  species  as  the  dandelions,  asters,  sunflowers,  mari- 
golds, and  chrysanthemums.  Very  few  of  the  species  of  Com- 
positae, however,  are  represented  in  the  spring  flora,  and  hence 
the  following  discussion  will  be  limited  to  a  single  species. 

ACHILLEA  MILLEFOLIUM  (YARROW) 

The  yarrow  is  a  perennial  herbaceous  plant  with  leaves  dis- 
sected into  narrow  segments  and  with  terminal  flower  clusters 
in  the  form  of  compound  corymbs.  It  is  common  in  spring  by 


HERBACEOUS  AND  WOODY  DICOTYLEDONS      411 

roadsides  and  in  waste  places  and  is  a  convenient  type  for  study. 
The  reproductive  structures  of  the  Compositae  are  so  similar  that 
when  one  type  is  mastered  the  variations  from  this  type  are 
easily  understood. 

Inflorescence.  The  flowers  of  the  yarrow  are  borne  in  dense 
heads,  each  surrounded  by  a  cluster  of  bracts,  which  together 
form  the  involucre  (Fig.  261,  A).  Each  separate  head  of  flowers 


FIG.  261.    Inflorescence  and  flowers  of  yarrow  (Acliillea) 

A,  single  head,  or  inflorescence;  B,  section  of  the  same;  C,  ray  flower;  D,  disk 

flower;' a,  anther;  ch,  bract;  d,  disk  flower;  o,  ovary;  re,  corolla  of  ray  flower; 

s,  stigma ;  tc,  corolla  of  tubular  flower 

x 

thus  represents  an  inflorescence  with  a  broad,  flat  receptacle  on 
which  the  small  flowers  are  closely  crowded.  The  outer  flowers 
bear  an  outgrowth  of  the  corolla  which  is  strap-shaped,  while 
in  the  central  flowers  the  corollas  are  tubular.  The  outer  strap- 
shaped  flowers  are  the  rav  flowers  of  the  head  and  serve  to 
render  it  more  conspicuous. 

Flowers.  The  flowers  are  all  epigynous,  with  the  floral  parts 
borne  above  and  upon  the  ovary,  which  is  composed  of  two  spo- 
rophylls,  or  carpels.  The  perianth  is  composed  of  a  corolla  the 


412 


GENERAL  BOTANY 


parts  of  which  are  united  to  form  a  tube.  The  calyx  is  absent 
in  the  yarrow,  but  in  many  other  Compositae  it  is  split  into  a 
large  number  of  fine  hairs  which  together  are  called  the  pappus. 
The  essential  organs  comprise  five  stamens  and  a  single  com- 
pound pistil  composed  of  two  sporophylls,  or  carpels.  The  ovary 
bears  a  single  seed  and  forms  an  achene  in  fruit.  The  style  splits 
at  the  apex  to  form  two  lobes,  the  inner  faces  of  which  are 
roughened  to  form  the  stigmatic  surfaces.  The  apex  of  each 


KJi/fc<^«rcrU/ 

\7       ^ 

t-fAnther  tube^r 

]r 

\U—  Filaments—  U 

if 

.Style  

-Corolla  tube- 

Ovary  1 

y 

' 

Stigma— 


FIG.  262.    Flower  and  fruit  of  the  yarrow  and  of  the  dandelion  (Taraxacum) 

a,  young  tubular  flower  of  yarrow,  with  stigma  still  within  the  stamen  tube ;  6,  older 

flower,  with  the  style  and  stigma  protruding ;  c,  ligulate  flower  of  the  dandelion  and 

its  parts ;  d,  fruit  with  pappus  and  beak 

stylar  lobe  is  also  furnished  with  a  brush  of  hairs  which  together 
constitute  the  stylar  brush.  The  stamens  are  attached  to  the 
corolla  by  slender  filaments,  and  the  anthers  are  united  to  form 
a  tube  around  the  style. 

Pollination.  The  umbellate  inflorescences  of  the  yarrow  form 
a  continuous  flat-topped  floral  mass  which  is  rendered  conspicu- 
ous by  the  white  or  pink  ray  flowers.  Since  there  may  be  as 
many  as  one  hundred  heads  in  the  entire  inflorescence,  with  from 
fifteen  to  twenty  flowers  in  each  head,  a  visiting  insect  is  certain 
to  come  in  contact  with  a  large  number  of  separate  flowers,  all 
of  which  are  especially  adapted  to  secure  cross-pollination.  The 
mechanism  of  cross-pollination  is  concerned  with  the  structure 


HERBACEOUS  AND  WOODY  DICOTYLEDONS      413 

of  the  flower  already  described  and  with  the  relation  of  the  anther 
tube  and  the  stigmatic  surfaces  in  young  and  old  flowers.  The 
flowers  are  protandrous,  and  in  young  flowers  the  pollen  is  shed 
while  the  style  is  short  and  the  stigmatic  surfaces  closely  approxi- 
mated (Fig.  262,  a).  As  the  flower  matures  the  style  elongates 
and  the  stylar  brush  pushes  the  pollen  .out  of  the  anther  tube, 
where  it  may  come  in  contact  with  an  insect's  body  passing  over 
the  flower.  When  the  end  of  the  style  and  the  stylar  brush 
emerge  from  the  corolla  tube,  the  two  halves  into  which  the  tip 
is  divided  separate,  and  the  stigmas  are  exposed  for  pollination 
with  pollen  from  another  flower  (Fig.  261,  D,  and  Fig.  262,  6,  c). 
In  case  cross-pollination  is  not  effected  self-pollination  may  be 
brought  about  by  pollen  falling  from  the  stylar  brush  upon  the 
outer  portion  of  the  stigmatic  surface  in  the  same  flower. 

Fruit.  The  fruit  is  an  achene,  that  is,  a  ripened  ovary  with 
one  inclosed  seed.  In  the  yarrow  there  is  no  special  device  for 
seed  dissemination,  but  in  many  of  the  Compositae,  such  as 
the  dandelion  (Taraxacum),  Spanish  needles  (Bidens),  and 
thistle  {Carduus),  special  outgrowths  in  the  form  of  the  pappus 
or  of  spines  facilitate  the  wide  dispersal  of  seeds  by  animals  or 
wind  (Fig.  262,  c,  d).  The  large  number  of  flowers  that  are  so 
perfectly  adapted  to  cross-pollination,  and  the  special  devices  for 
disseminating  the  seeds,  enable  the  Compositae  to  spread  and  to 
increase  with  extreme  rapidity  wherever  opportunity  is  offered. 

4 

DICOTYLEDONS  AND  MONOCOTYLEDONS 

The  angiosperms  which  we  have  studied  thus  far  have  all 
belonged  to  the  dicotyledons,  or  plants  like  the  bean,  mandrake, 
and  marigold,  which  have  two  cotyledons  in  the  embryo.  The 
monocotyledons  have  but  one  cotyledon  in  the  embryo,  —  a 
condition  which  is  supposed  to  have  arisen  by  the  loss  of  one 
cotyledon  in  the  ancestral  dicotyledons.  The  close  relationship 
of  the  two  groups  is  further  emphasized  by  the  great  similarity 
between  them  in  the  structure  of  the  flower  and  in  the  game- 
tophyte  plants  resulting  from  the  germination  of  the  spores,  and 
also  by  the  occurrence  of  species  with  intermediate  characters. 


GENERAL  BOTANY 


The  original  distinction  between  the  monocotyledons  and  the 
dicotyledons  was  based  upon  the  number  of  cotyledons  in  the 
embryo,  as  the  names  signify,  since  the  monocotyledons  were 
known  to  have  embryos  with  but  one  cotyledon,  while  the 


Parallel} 
veined 

leaf] 


ISlem  — *-^/ 

(bundles  scattered)    ( vascular  cylinder}  Belted-veined  leaf 

*>) 


Flower' 


Floral  plan.          FloraTplan 
of  three  Offive 


'Seed 


Cotyledon-/.] 

'Stem  apex. .1  I 
•Hypocotyl-\  I 

Embryo 
(monocotyledon} 


| Cotyledon- 

Stem  apex^ 

— -Hypocotyl- 


Enwryo' 
(dicotyledon) 


Seed 


FIG.  263.   Comparison  of  monocotyledons  and  dicotyledons 

a,  vegetative  and  reproductive  structures  of  a  monocotyledon ;  6,  similar  structures 

of  a  dicotyledon 

embryos  of  dicotyledons  had  two  cotyledons,  as  in  the  common 
beans  and  peas.  The  mature  plants  of  the  two  groups  also 
manifest  quite  marked  distinctions,  which  apply  to  the  form 
and  venation  of  the  leaves,  the  structure  of  the  stem,  and  the 
numerical  plan  of  the  flowers  (Fig.  263). 

The  leaves  of  monocotyledons  are  usually  linear  with  parallel 
venation,  like  the  leaves  of  lilies  and  grasses.    The  venation  is 


HERBACEOUS  AND  WOODY  DICOTYLEDONS      415 

also  designated  as  closed  venation,  since  the  veins  do  not  termi- 
nate in  the  margin  of  the  leaves,  which  therefore  remain  smooth. 
In  dicotyledons  the  leaves  are  usually  netted-veined,  and  the 
veins  end  free  in  a  rough  margin. 

The  stems  of  monocotyledons  also  have  scattered  vascular 
bundles  without  a  cambium  layer,  while  those  of  dicotyledons 
form  a  cylinder  in  which  the  cambium  adds  new  tissue  to  the 
phloem  and  xylem  layers  annually. 

The  flowers  of  the  monocotyledons  are  habitually  on  the  plan 
of  three,  while  dicotyledonous  flowers  are  more  frequently  on  the 
plan  of  four  or  five  parts  in  a  whorl  for  each  set  of  floral  organs. 
These  distinctions  between  dicotyledons  and  monocotyledons  are 
graphically  illustrated  in  Fig.  263  and  are  concisely  stated  in  the 
summary  below. 

SUMMARY 

The  embryo  of  monocotyledons  has  but  one  cotyledon  and  a  lat- 
eral stem  tip,  while  dicotyledons  have  two  cotyledons  and  a  central 
stem  tip. 

The  monocotyledons  usually  have  leaves  with  parallel" veins,  and 
the  veinlets  do  not  end  free  in  the  margin  of  the  leaf,  as  in  dicoty- 
ledons, in  which  netted  venation  prevails. 

In  the  stems  of  monocotyledons  the  vascular  bundles  are  scattered. 
In  dicotyledons  the  xylem  and  phloem  form  a  cylinder  inclosing  the 
central  pith. 

The  flowers  of  monocotyledons  are  in  threes,  but  those  of  dicoty- 
ledons are  in  fours  or  fives. 


CHAPTER  XXI 

MONOCOTYLEDONS 

Habitat  and  habit.  The  monocotyledons  include  over  forty 
known  families  and  about  twenty-five  thousand  species  of  plants, 
the  greater  number  of  which  are  water-loving,  being  either  true 
hydrophytes,  like  the  pondweeds  (Pontederia  and  Potamogetori), 
or  semiaquatics,  like  the  marsh  grasses  and  sedges.  A  few  forms, 
such  as  the  yuccas  and  desert  grasses,  are  xerophytic,  while  a 
large  number  should  be  classed  as  tropophytes,  adapted  to  alter- 
nating seasons  of  moisture  and  drought.  Among  these  plants  are 
many  of  the  wild  and  cultivated  species  in  which  the  underground 
stem  takes  the  form  of  a  rhizome,  bulb,  or  corm,  which  enables 
the  plant  to  live  securely  underground  during  an  inclement 
season.  The  favorable  season  with  such  plants  is  used  for  the 
growth  of  aerial  stems  bearing  leaves  which  make  food,  and  for 
the  production  of  flowers,  fruit,  and  seeds.  The  majority  of  these 
species  have  the  characters  already  ascribed  to  monocotyledons, 
namely,  narrow,  parallel-veined  leaves,  aerial  stems  with  scattered 
bundles,  and  flowers  on  the  plan  of  threes. 

Reproduction  and  seasonal  life.  The  monocotyledons  with 
underground  stems  reproduce  sexually  by  means  of  flowers  and 
vegetatively  by  means  of  corms,  bulbs,  runners,  or  tubers  formed 
as  offshoots  from  the  mother  plants.  Vegetative  reproduction 
facilitates  local  increase  in  the  immediate  vicinity  of  the  mother 
plant,  while  seeds,  formed  as  a  result  of  sexual  reproduction, 
facilitate  the  distribution  of  the  species  over  wide  areas.  The 
seasonal  life  is  therefore  much  the  same  as  that  of  the  white 
sweet  clover  and  of  the  perennials  outlined  in  the  summary  of 
the  seasonal  life  of  plants  in  Part  I. 

The  commercial  importance  of  the  monocotyledons  is  due  to 
the  great  beauty  of  many  species,  such  as  the  lilies,  tulips,  and 
hyacinths,  and  to  the  value  of  other  species  for  food  and  forage. 

416 


MONOCOTYLEDONS 


417 


COMMELINACEAE   (SPIDERWORT  FAMILY) 
TRADESCANTIA  (SPIDERWORT) 

Habitat  and  habit.  The  spiderworts  are  among  the  commonest 
wild  and  cultivated  plants  of  the  spring  and  summer  flora.  They 
usually  inhabit  gravelly,  sandy,  or  alluvial  soils  in  woods  or  along 
railroads  and  river 
banks.  In  Trades- 
cantia  virginiana 
(Fig.  265)  the  stem 
is  both  aerial  and 
subterranean.  The 
aerial  stem  bears 
the  long,  parallel- 
veined  leaves  and 
blue  flowers  char- 
acteristic of  the 
species.  The  un- 
derground stem  is 
tuberous  and  gives 
rise  to  one  or  more 
lateral  flowering 
shoots  by  means 
of  lateral  buds.  It 
also  enables  the 
plant  to  live  over 
the  late  summer, 
autumn,  and 


Vascular 
bwndle 


A,  plant  in  flower;  B,  reproductive  structures  and  stem 
section  (a,  front  view  of  flower;  b,  ground  plan;  c,  dehis- 
cent fruit ;  d,  section  of  stem) ;  C,  leaf  and  parallel  venation 


FIG.  264.   Habit  and  reproduction  of  the  common 

spiderwort  ( Tradescantia) 
win- 
ter, furnished  with 
buds   and    reserve 
food  for  the  early  spring  growth.    Tradescantia  is  thus  a  semi- 
xerophyte  or  semitropophyte,  like  many  of  the  cultivated  bulbous 
monocotyledons. 

Reproduction.  The  blue  or  purple  flowers  are  ephemeral,  that 
is,  lasting  for  a  day  only,  and  have  their  parts  arranged  in  threes 
(Fig.  264,  J5)  like  other  typical  monocotyledons, 


418 


GENERAL  BOTANY 


Pollination  is  by  means  of  insects,  which  visit  the  flowers  in 
great  numbers  and  pollinate  the  stigma,  which  protrudes  beyond 
the  anthers  and  hairy  filaments.  The  fruit  is  dry  and  opens  by 

three  valves,  repre- 
senting the  three  car- 
pels of  the  ovary,  and 
liberates  the  smooth 
black  seeds  for  dis- 
semination. Thus  the 
plant  reproduces  in 
two  ways,  —  sexually 
by  means  of  flowers, 
fruits,  and  seeds,  and 
vegetatively  by  means 
of  tubers  with  lateral 
buds,  which  very  often 
produce  underground 
branches  of  consider- 
f---f$  able  length. 

LlLIACEAE  (LlLY 

FAMILY) 

The  Liliaceae  are 
of  principal  interest 
on  account  of  the 
large  number  of  or- 
namental plants  in- 
cluded among  them, 
although  food  plants, 
such  as  the  onions 
and  asparagus,  are 
also  members  of  this 
important  family.  The  ornamental  plants  include  the  common 
cultivated  lilies,  the  spring  tulips,  hyacinths,  narcissus,  and 
lily  of  the  valley.  Among  the  wild  species  of  greatest  beauty 
are  the  dogtooth  violet  (Erythronium),  the  bell  wort  (  Uvularia), 


FIG.  265.   Lengthwise  section  of  hyacinth  plant 

st,  stem;   6,  young  bulb  for   next  year's   growth 
sc,  bulb  scales ;  fs,  flower  stalk.    Reduced 


MONOCOTYLEDONS  419 

Trillium,  false  Solomon's  seal  {SmilacincC),  and  Ornithogalum 
(Fig.  266).  Most  of  these  plants  spring  from  bulbs,  tubers,  or 
rhizomes,  and  so  are  adapted  to  a  tropophytic  existence  and  a 


Courtesy  of  the  American  Magazine  of  Forestry,  Washington,  D.  C. 

FIG.  266.    Ornithogalum,  star  of  Bethlehem 
Natural  habitat  and  habit  of  a  flower  in  bloom.   Photograph  by  Dr.  R.  W.  Shufeldt 

corresponding  seasonal  life.  Many  of  the  cultivated  species  origi- 
nated in  arid  regions,  where  the  short  rainy  season  is  followed 
by  a  long  dry  period,  as  in  California  or  the  Mediterranean  region. 
In  such  habitats  the  underground  stem  enables  the  plant  to  live 
securely  during  the  dry  season,  while  the  great  store  of  food  in 


420  GENERAL  BOTANY 

the  bulbs,  tubers,  or  rhizomes  facilitates  the  rapid  growth  of 
aerial  shoots  and  flowers  during  the  short  rainy  season.  In  tem- 
perate regions  the  wild  species  often  inhabit  hillsides  or  banks 
where  dry  conditions  are  imposed  during  the  summer  months. 
Commercial  importance.  This  habit  of  producing  underground 
stems  facilitates  the  cultivation  of  the  Liliaceae  for  commercial 
purposes,  since  the  bulbs,  corms,  and  rootstocks  thus  produced 
allow  of  shipments  and  long  storage,  which  would  otherwise  be 
impossible.  Tulips  and  hyacinths,  for  example,  are  grown  in 
Holland  for  shipment  to  this  country.  The  bulbs  arise  as  lateral 
buds  which  produce  the  young  bulbs  or  offsets  in  considerable 
numbers  each  year  from  the  mother  bulbs  (Fig.  79).  Ten  or 
twelve  daughter  bulbs  are  produced  from  one  mother  tulip  bulb 
in  a  season,  and  as  many  as  twenty  or  more  from  a  hyacinth 
bulb.  These  offsets  are  grown  for  from  three  to  five  years,  then 
dried  and  shipped  to  various  parts  of  the  world  for  the  growth 
of  flowers  for  ornamental  and  decorative  purposes.  The  biologi- 
cal and  commercial  history  of  the  narcissus,  lilies,  onions,  and 
other  ornamental  and  food-producing  varieties  of  the  lily  family 
is  very  similar  to  that  briefly  sketched  for  the  tulip  and  the  hya- 
cinth. Most  of  these  species  have  been  greatly  improved,  as 
regards  variation  in  color  and  size,  by  hybridization ;  but  the 
perpetuation  of  such  variation  must  be  secured  by  vegetative 
reproduction,  as  we  learned  earlier  in  the  study  of  reproduction, 
hybridization,  and  breeding. 

SMIL  AGIN  A  AND  EEYTHEONIUM 

Habitat  and  habit.  The  species  of  Smilacina  usually  inhabit 
wooded  slopes  in  the  shade  of  trees,  where  a  considerable  amount 
of  humus  is  present  in  the  soil.  They  represent  those  species  of 
the  Liliaceae  which  have  both  an  underground  and  an  aerial  stem, 
with  the  usual  differentiation  in  function  between  the  two  kinds 
of  stems.  The  aerial  stem  bears  the  leaves  and  the  cluster  of 
flowers  of  the  summer  season,  while  the  underground  stem  serves 
for  food  storage,  conduction,  and  the  formation  of  buds  for  the 
next  season's  growth  (Fig.  267,  A).  During  dry  periods,  and  in 


422 


GENEKAL  BOTANY 


the  winter  months,  the  underground  rhizome  enables  these  plants 
to  live  protected  from  any  danger  of  drought  or  destruction  by 
freezing.  In  the  spring  the  young  buds  and  growing  roots  are 


FIG.  269.   Stages  in  the  development  of  Erythronium 

First  year,  germinating  seed  and  seedling ;  second  and  third  years,  first  bulb ;  fourth 

and  fifth  years,  new  bulbs  being  formed  deeper  in  the  soil ;  sixth  and  seventh  years, 

larger  bulbs  and  plants.   From  Bergen  and  Caldwell's  "Practical  Botany  " 

furnished  with  an  abundance  of  food  stored  up  during  the 
previous  season  in  the  rhizome  and  now  digested  and  circulated 
for  use  in  spring  growth. 

Erythronium  (Fig.  268)   occupies  much  the  same  habitat  as 
Smilacina  and  has  the  same  general  habit  and  seasonal  life.    The 


MONOCOTYLEDONS  423 

underground  stem  is  in  the  form  of  a  solid,  scaly  bulb  from 
which  two  characteristically  spotted  leaves  and  a  single  flower 
scape  grow  each  spring.  The  most  distinctive  feature  of  Erythro- 
nium  is  the  peculiar  method  by  which  the  bulbs  of  each  season 
become  more  deeply  buried  in  the  soil.  The  new  bulbs  are 
formed  at  the  ends  of  runners  arising  from  buds  in  the  axils  of 
the  scale  leaves.  The  runners  of  each  season  (Fig.  269)  grow 
downward  and  so  bury  each  new  daughter  bulb  a  little  deeper  in 
the  soil  than  the  mother  bulb.  Six  or  seven  years  are  necessary 
for  the  production  of  a  bulb  strong  enough  to  bear  flowers.  The 
seasonal  history  of  Erythronium  and  Smilacina  is  therefore  not 
unlike  that  of  the  tulip  and  hyacinth  or  of  a  perennial  woody 
plant  in  which  a  long  period  of  vegetative  activity  is  necessary 
before  reproduction  by  flowers  and  fruit  is  possible. 

Reproduction.  The  flowers  in  Smilacina  and  Erythronium  differ 
from  those  of  Tradescantia  in  that  both  whorls  of  the  perianth 
are  colored  like  a  corolla  and  are  therefore  not  differentiated 
into  a  distinct  calyx  and  corolla.  This  feature  is  also  character- 
istic of  the  cultivated  lilies,  tulips,  and  hyacinths,  to  which 
Smilacina  and  Erythronium  are  closely  related.  Cross-pollination 
is  effected  by  insects,  since  the  stamens,  when  ripe,  are  shorter 
than  the  pistil,  so  that  the  stigma  protrudes  beyond  the  anthers. 
It  is  thus  in  a  position  to  receive  pollen  from  insects  which 
have  recently  visited  other  flowers. 

The  fruit  in  Smilacina  (Fig.  267,  C)  is  a  berry,  and  the  seeds 
are  disseminated  when  the  berry  disintegrates,  or  they  may  be 
distributed  by  animals  which  eat  the  fruit.  In  Erythronium  the 
fruit  is  a  capsule  which  splits  into  three  valves  corresponding 
to  the  three  carpels  of  the  ovary,  thus  disseminating  the  seeds. 

IRIDACEAE  AND  ARACEAE  (!RIS  AND  ARUM  FAMILIES) 

The  species  of  these  two  families  are  of  particular  interest  on 
account  of  the  special  adaptations  of  the  flowers  for  securing 
cross-pollination  by  insects.  Their  vegetative  characteristics  are 
also  of  biological  importance  as  indicating  the  characteristic  habit 
of  the  aerial  and  underground  parts  of  the  monocotyledons 
already  emphasized  in  Tradescantia  and  Liliaceae. 


424  GENERAL  BOTANY 

IRIDACEAE  (!EIS  FAMILY) 
IRIS  VERSICOLOR  (COMMON  BLUE  FLAG) 

Habitat  and  habit.    The  common  wild  blue  flag  of  the  spring 
flora  inhabits  swampy  land  on  the  borders  of  streams  and  lakes 

canal. 


FIG.  270.  Flower  and  reproductive  structures  of  the  iris  (Iris  versicolor) 
A,  flower  and  its  parts ;  B,  ovary,  style,  and  stigma ;  (7,  pollen  tubes 

and  is  thus  adapted  to  a  hydrophytic  habitat.  Like  many  plants 
of  such  habitats,  however,  its  habit,  represented  by  the  form  and 
structure  of  its  leaves  and  underground  stem,  is  that  of  a  xero- 
phyte.  This  is  probably  due  to  the  large  percentage  of  organic 
matter,  including  organic  acids,  in  the  water  of  the  soil  which 
surrounds  the  roots  of  these  plants  in  the  marshes  and  swamps 
where  they  live. 

The  short  aerial  stem  bears  characteristic  monocotyledonous 
leaves  and  highly  organized  flowers. 


MONOCOTYLEDONS  425 

The  cultivated  forms  of  Iris  have  a  similar  habit  and  also 
a  similar  floral  structure,  which  has  become  highly  modified  by 
hybridization  and  selection  with  a  view  to  increasing  the  size, 
color,  and  beauty  of  the  flowers. 

Reproduction.  The  flower  (Fig.  270,  A)  is  epigynous,  with  the 
floral  parts  arranged  in  sets  of  three  each.  The  outer  lobes  of 
the  perianth  are  large  and  highly  colored,  while  the  inner  ones 
are  smaller  and  less  conspicuous.  The  three  stamens  spring  from 
the  base  of  the  outer  perianth  lobes,  and  each  stamen  is  beneath 
a  branch  of  the  three-lobed  style.  The  stigmas  are  on  the  upper 
surface  of  a  flap  which  grows  out  from  each  stylar  lobe.  When 
an  insect  visits  the  flower  for  nectar,  it  comes  to  rest  upon  one 
of  the  outer  lobes  of  the  perianth  and  then  crawls  down  into  the 
flower  to  probe  for  nectar  at  the  base  of  the  perianth.  In  so  doing 
it  dusts  pollen  onto  its  back,  which  cannot  reach  the  stigmatic 
surface  as  it  crawls  out,  but  is  in  a  position  to  pollinate  a  stigma 
of  the  next  flower  visited.  The  pollen  germinates  quickly  on  the 
stigma  of  Iris,  and. the  pollen  tubes  follow  down  the  stylar  canal 
in  the  center  of  the  style  to  the  ovules,  where  fertilization  takes 
place.  The  flower  is  thus  admirably  adapted  to  cross-pollination 
by  insects. 

ARACEAE  (ARUM  FAMILY) 

AEISAEMA  (JACK-IN-THE-PULPIT) 

Habitat  and  habit.  The  common  jack-in-the-pulpit  (Arisaema 
triphyllum)  is  one  of  the  most  familiar  representatives  of  the  Arum 
family  in  the  spring  flora.  It  grows  naturally  in  moist  humus  soil 
in  the  shade  of  trees  and  is  thus  mesophytic  in  habit  and  habitat. 

The  aerial  stem  bears  one  or  two  leaves  and  peculiar  greenish 
flowers.  The  underground  stem  is  a  turnip-shaped  corm  which 
bears  the  annual  aerial  shoot  from  a  terminal  bud. 

The  seasonal  history  of  the  jack  is  an  interesting  one  and  will 
serve  as  another  illustration  of  the  peculiar  adaptations  of  some 
of  the  monocotyledons  with  underground  stems  for  perpetuat- 
ing themselves  by  vegetative  reproduction.  The  new  roots  arise 
each  year  from  the  upper  part  of  the  corm  (Fig.  271,  A),  while 


426 


GENERAL  BOTANY 


the  nourishment  is  stored  in  the  base  of  the  corm  for  the  early 
growth  of  the  bud  which  produces  the  annual  aerial  stem.  The 
new  corm  is  therefore  formed  above  the  old  corm  each  season, 
and  the  tissues  of  the  latter  disappear  as  the  new  corm  is 
formed  above  on  its  remains.  Large  conns  also  produce  lateral 

buds,  similar  to 
those  of  the  gladio- 
lus (Fig.  80),  which 
give  rise  to  a  circu- 
lar cluster  of  corms 
around  the  mother 
corm. 

Sexual  reproduc- 
tion. The  flowers 
are  borne  on  a 
fleshy  axis  called 
the  spadix,  which 
is  included  in  a 
bractlike  structure 
called  the  spathe 
(Fig.  271,  B,  a). 
The  entire  struc- 
ture is  often  mis- 
taken for  a  flower, 
although  it  is  really 
an  inflorescence. 

Cross-pollination 
is  assured,  since 
male  and  female 


•Anther 


FIG.  271.    Habit  and  flower  of  jack-in-the-pulpit 
(Arisaema) 

A,  plant  with  flowers  and  corm ;  B  (a,  spathe,  spadix,  and 

staminate  flowers ;  6,  cluster  of  stamens;  c,  pistil).  Copied 

from  Curtis's  "  Nature  and  Development  of  Plants  " 


flowers  are  usually  borne  on  separate  plants.  The  male  plants 
are  also  smaller,  as  a  rule,  than  the  female  plants,  which  is -an 
advantage,  since  the  female  plants  must  produce  seed  and  fruit 
and  so  need  the  great  store  of  reserve  food  contained  in  the 
larger  corms.  The  production  of  pistillate  flowers  from  the  larger 
corms  is  supposed  to  be  connected  with  the  abundant  food 
supply.  The  fruit  is  composed  of  a  cluster  of  beautiful  red 
berries  borne  on  the  lower,  fleshy  portion  of  the  spadix. 


MONOCOTYLEDONS 


427 


ORCHIDACEAE  (ORCHID  FAMILY) 
CYPRIPEDIUM  (LADY'S-SLIPPER,  OR  MOCCASIN  FLOWER) 

Habitat  and  habit.   The  flowers  of  the  lady's-slipper  belong  to 
the  orchid  family  (the  flowers  of  which  are  famous  for  their  great 


Courtesy  of  the  American  Magazine  of  Forestry,  Washington,  B.C. 


FIG.  272.    The  yellow  lady's-slipper 
(Cypripedium  pubescens) 

Photograph  by  Dr.  R.  W.  Shufeldt 


FIG.  273.  The  pink  lady's-slipper 
(Cypripedium  acaule) 

Photograph  by  Dr.  R.  W.  Shufeldt 


beauty  as  conservatory  plants)  and  are  biologically  interesting 
on  account  of  their  wonderful  mechanisms  for  securing  cross- 
pollination  by  means  of  insects.  The  plants  inhabit  moist,  shady 
woods  with  soil  containing  plenty  of  humus,  or,  in  the  case  of 
some  species,  low,  marshy  regions  along  streams,  ponds,  and 
lakes.  Their  habit  is  sufficiently  illustrated  in  the  text  figures 


428 


GENERAL  BOTANY 


of  the  common  yellow  lady's-slipper  (Cypripedium  pubescens) 
(Fig.  272)  and  the  pink  species  (Cypripedium  acaule)  (Fig.  273). 
^  Reproduction.  The  flowers  of  the  lady's-slipper  are  simpler 
than  those  of  the  true  orchids,  but  are  nevertheless  very  highly 
modified  in  such  a  manner  as  to  prevent  self-pollination.  One 
of  the  petals  is  developed  into  a  saclike  structure  (the  labellum, 

or  lip)  observed  in  the 
figures,  with  a  narrow  en- 
trance above  for  insects. 
The  style  is  highly  modi- 
fied and  projects  into  this 
opening  to  the  labellum, 
bearing  the  stigma  and 
two  stamens  on  its  under- 
side (Fig.  274).  Insects 
make  their  way  into  the 
cavity  of  the  labellum  on 
either  side  of  the  project- 
ing style.  In  order  to  get 
out  they  are  obliged  to  rub 
against  the  stamens  and 

o,  ovary  ;  a,  anther  of  a  perfect  stamen  ;  sta,  im-      are  thus  dusted  with  pol- 
perfect  stamen  :  stig,  stigma  ,         ,™  .  , 

len.  The  stigma  is  situated 

below  the  stamens,  so  that  the  outgoing  insect  does  not  pollinate 
it.  The  next  flower  visited,  however,  is  certain  to  receive  on 
its  stigma  the  pollen  from  the  flower  previously  visited. 


FIG.  274.   Vertical  section  of  a  flower  of 
Cypripedium  acaule 


ALISMACEAE  AND  PONTEDERIACEAE 
SAGITTARIA  AND  PONTEDERIA 

Habitat  and  habit.  Sayittaria  (arrowhead)  and  Pontederia 
(pickerel  weed)  are  good  examples  of  Jiydrophytic  monocotyle- 
dons which  inhabit  shallow  water  on  the  margins  of  lakes  or 
streams.  In  Sagittaria  the  flowering  stem  and  leaves  arise  from 
stolons  which  are  buried  in  the  mud  at  the  bottom  of  the  lake 
or  stream.  In  Pontederia  the  underground  stems  are  rhizomes 
which  give  rise  to  a  flower-bearing  shoot  and  long-petioled  leaves. 


8  M 

to  « 

'I  'ft 

a  « 


Ssi      tM 

I    ° 


t 


430 


GENERAL  BOTANY 


Reproduction.  In  Sagittaria  (Fig.  275)  the  flowers  are  borne 
in  clusters  of  three  on  the  inflorescence  axis,  while  in  Pontederia 
(Fig.  276)  they  occur  in  spikes.  Cross-pollination  is  provided 
for  in  Sagittaria,  since  plants  are  either  moncecious  or  dioecious. 


FIG.  277.   A  field  of  sugar  cane  at  Vera  Cruz 
After  Freeman  and  Chandler 

In  Pontederia  the   flowers    are    trimorphic,  with  three   lengths 
of  stamens  and  pistils,  which  also  insures  cross-pollination. 

G-RAMINEAE   (GRASSES  AND  SEDGES) 

FORAGE  AND  FOOD  PLANTS 

Habitat  and  habit.  The  members  of  this  family,  including  the 
common  grasses,  bamboo,  sugar  cane,  and  cereal  grains,  are  among 
the  most  important  and  widely  spread  of  all  the  monocotyledons. 
The  grasses  proper  are  social  plants,  forming  vast  associations 


MONOCOTYLEDONS 


431 


comprising  the  principal 
vegetation  of  the  meadows, 
plains,  marshes,  and  slopes 
of  this  and  other  countries. 
They  are  therefore  either 
mesophytic,  hydrophytic,  or 
xerophytic. 

Many  of  the  grasses  per- 
petuate and  spread  the 
species  by  means  of  under- 
ground stems  in  the  form 
of  either  rootstocks  or  stolons, 
which  enable  them  to  form 
dense  mats  and  sods  wher- 
ever they  gain  a  foothold 
(Fig.  278).  The  leaves  are 
characteristic  of  monocoty- 
ledons generally,  being  long 
and -strap-shaped,  with  par- 
allel veins.  The  flowers  are 
highly  modified,  and  the 
fruit  in  the  true  grasses 
and  cereal  grains  is  an 
achene,  known  as  the  cary- 
opsis,  or  grain. 


FIG.  278.    Habit  of  the  couch  grass,  a  weed  pest 

Note  that  the  aerial  stems  spring  from  nodes  of  the  underground  rhizome.  When  the 
rhizome  is  cut  in  pieces,  each  node  can  reproduce  a  new  plant  and  so  spread  the  weed 

Economic  importance.  The  true  grasses  are  of  the  greatest 
importance  in  furnishing  pasturage  and  hay  for  animals  and  in 
providing  a  good  turf  for  lawns  and  meadows.  The  cereal  grains, 
including  corn,  wheat,  oats,  and  rye,  are  all  grasses  which  have 


432 


GENERAL  BOTANY 


been  cultivated  and  greatly 
improved  by  man.  Corn, 
for  instance,  is  supposed  to 
have  been  derived  from  a 
wild  grass  or  to  be  a  hybrid 
between  two  grasslike  an- 
cestors. The  wild  wheats 
of  Palestine,  from  which 
the  cultivated  varieties  are 
supposed  to  have  come,  are 
essentially  grasses  in  which 
the  fruit  is  a  grain  of  the 
greatest  value  to  man  for 
flour.  All  the  other  cereals 
have  originated  similarly 
from  the  grasses  and  have 
been  gradually  improved 
by  methods  of  culture  de- 
scribed in  the  chapters 
on  hybridization,  selection, 
and  evolution. 

In  addition  to  the  grasses 
and  cereals  the  grass  fam- 
ily includes  sugar  cane 
(Fig.  277)  (used  for  mak- 
ing sugar),  bamboo  (used 
for  fishing  rods),  and  rice 
(used  as  a  food  plant). 

Reproduction.  The  flow- 
ers of  the  grasses  are  usu- 
ally very  highly  modified, 
and  their  relation  to  the 
flowers  of  the  monocoty- 
ledons thus  far  studied  is 
difficult  to  determine.  In 
the  corn  plant  (Fig.  102)  the  flowers  are  borne  separately,  the 
staminate  flowers  forming  a  compound  inflorescence,  known  as 


FIG.  279.    Structure  of  an  ear  of  corn,  or 
pistillate  inflorescence 

A,  section  of  a  young  ear,  showing  the  cob,  or 
axis  of  inflorescence  (ax),  and  the  silk,  or  style, 
and  stigmas  (si) ;  B,  ovary,  showing  ovule  (o), 
and  style ;  C,  upper  portion  of  style  (silk)  and 
stigmas,  enlarged 


MONOCOTYLEDONS  433 

the  tassel,  and  the  pistillate  flowers  forming  an  entirely  different 
kind  of  inflorescence,  known  as  the  ear  (Fig.  279).  Cross- 
pollination  is  almost  certain  in  corn,  although  self-pollination  is 
not  impossible  where  the  stamens  and  pistils  ripen  together.  The 
anthers  on  a  single  corn  plant  are  said  to  produce  as  many  as 


Floral  plan  of  spikelet 

e 


FIG.  280.   Inflorescence  and  flower  of  the  oat  (Avena  saliva) 

a,  portion  of  an  oat  panicle,  or  inflorescence  ;  b,  c,  d,  different  views  of  the  oat 
spikelet  and  its  parts;  e,  floral  plan  of  spikelet 

50,000,000  pollen  grains,  so  that  complete  wind  pollination  of 
the  many  stigmas  constituting  the  silk  of  the  ear  is  practically 
assured.  Where  pollination  does  not  occur,  the  seeds  do  not 
develop. 

The  flowers  of  the  true  grasses  and  cereal  grains  are  usually 
borne  in  dense  spikes,  like  those  of  wheat  and  timothy,  or  in 
compound  clusters  made  up  of  separate  spikelets,  like  the  panicle 
of  oats  (Fig.  280,  a).  The  spike,  like  the  panicle,  is  composed 
of  several  spikelets,  each  spikelet  containing  one  or  more  flowers 
(Fig.  280,  b-e).  The  flower  proper  (Fig.  281)  is  greatly  reduced, 


434 


GENERAL  BOTANY 


being  composed  of  a  pistil  with  two  plumose  stigmas,  three 
stamens,  and  two  small  rudimentary  organs  at  the  base  of  the 
pistil,  cajled  lodicules.  Each  flower  is  usually  surrounded  by  two 
pairs  of  bracts.  The  outer  bracts  are  called  glumes;  the  inner 
bracts  are  called  the  palet  and  the  lemma.  Botanists  are  almost 
unanimous  in  regarding  the  flower  of  the  grasses  as  a  highly 
modified  form  of  monocotyledonous  flower,  like  that  of  Trades- 
cantia  or  the  lily,  in  which  the  perianth  is  represented  by  the 
rudimentary  lodicules.  The  lodicules,  where  they  are  well  devel- 
oped, serve  to  open  the  glumes,  palet,  and  lemma,  so  as  to 


Glume 


Ovar 


Glume 


Flower 
a 


Floral  plan 

Flower  of  flower 

b  c 

FIG.  281.   Flower  of  the  oat 
a,  &,  two  views  of  the  flower  and  its  parts ;  c,  ground  plan  of  the  flower 

expose  the  essential  organs.  Where  the  lodicules  are  not  well 
developed,  the  flowers  remain  closed.  Cross-pollination,  where 
this  occurs,  is  usually  effected  by  the  wind,  the  anthers  being 
attached  to  the  filaments  at  the  middle,  so  as  to  be  readily 
swayed  to  and  fro  for  the  scattering  of  pollen.  In  many  of  the 
cereal  grains,  like  wheat  and  oats,  the  opening  of  the  flowers 
and  the  act  of  pollination  occur  early  in  the  day  and  are  often 
completed  in  a  relatively  short  time. 

The  fruit  is  the  caryopsis,  or  grain,  which  is  formed  by  a 
union  of  the  ovary  wall  with  the  seed  coat,  making  a  dry 
indehiscent  fruit. 


CHAPTER  XXII 

PLANT  ASSOCIATIONS 

In  the  previous  chapters  plants  have  been  considered  as  sep- 
arate individuals  or  as  groups  of  closely  related  individuals 
known  as  species.  In  nature,  however,  each  plant  is  a  member 
of  some  plant  community,  living  together  with  other  plants  which 
vitally  affect  its  growth  and  development.  The  student  will  also 
find  upon  investigation  that  the  vegetation  of  each  region  is  not 
uniform  in  character,  but  is  made  up  of  smaller  units,  such  as 
meadow,  pond,  and  cultivated  field  and  lawn,  each  of  which  is 
composed  of  plants  requiring  similar  conditions  of  soil  and  cli- 
mate for  their  best  development.  It  will  be  found  that  these 
various  plant  communities,  known  as  plant  associations,  although 
they  seem  to  the  uncritical  observer  to  be  stable,  are  never- 
theless constantly  changing,  owing  to  competition  among  the 
existing  members,  to  the  advent  of  new  species  by  migration,  or 
to  changes  in  the  earth's  surface  caused  by  fires,  floods,  and 
other  agencies,  which  have  a  far-reaching  effect  on  vegetation. 

Kinds  of  plant  associations.  The  most  general  classification 
of  plant  associations  is  that  mentioned  in  the  first  part  of  the 
text,  namely,  the  mesophytic  (Fig.  282),  Kydrophytic  (Fig.  286), 
and  xerophytic  (Fig.  71)  plant  associations,  based  upon  the  avail- 
able water  in  the  soil.  To  this  general  classification  a  fourth 
should  be  added,  to  include  the  halophytic  plant  associations, 
which  inhabit  salt  marshes  and  ponds  where  the  amount  of 
various  salts  in  the  water  is  excessive.  These  larger  associations 
are  usually  subdivided  into  smaller  ones,  which  reflect  more 
accurately  the  particular  environmental  conditions  prevailing  in 
particular  habitats. 

Thus,  the  hydrophytic  association  comprises  smaller  pond,  lake, 
stream,  and  swamp  associations,  which  differ  either  in  the  kinds 

435 


436 


GENERAL  BOTANY 


of  species  composing  them  or  in  the  proportion  existing  between 
the  constituent  species  of  each  minor  association.  In  a  similar 
manner,  the  xerophytic  associations  include  lesser  desert,  dune, 
cliff,  bog,  and  saline  plant  associations,  while  mesophytic  plants 
form  distinct  associations  in  the  form  of  forests,  meadows,  prairies, 
and  cultivated  fields.  These  large  and  small  plant  associations 


FIG.  282.  A  new  mesophytic  forest  association  (twenty-five  years  old)  of  honey 
locust,  white  elm,  and  black  walnut 

Photograph  furnished  hy  the  United  States  Forest  Service 

form  the  units  which  comprise  the  vegetation  of  local  areas  and, 
finally,  of  the  entire  land  surface  of  the  globe  occupied  by  plants. 

Origin  of  new  associations.  In  order  to  understand  the  impor- 
tant phenomena  connected  with  the  origin  and  development  of 
plant  associations  it  will  be  necessary  to  consider  certain  dynamic 
aspects  of  plant  life,  including  the  migration  of  plants  from  one 
locality  to  another,  the  invasion  and  occupation  of  new  territory 
by  such  migrants,  and  the  replacing  of  one  plant  population  by 
another  in  new  regions. 

It  is  evident  that  the  migration  of  plants  by  means  of  mobile 
seeds,  fruits,  or  other  reproductive  parts  must  be  a  potent  factor 


PLANT  ASSOCIATIONS 


437 


in  the  formation  of  new  plant  communities  in  denuded  areas  or 
in  repopulating  old  ones  where  space  still  remains  for  the  intro- 
duction of  new  individuals  or  species.  It  is  clear  also  that  the 
rapidity  with  which  any  given  plant  will  invade  such  areas  will 
depend  in  part  upon- the  nature  of  the  device  which  it  possesses 
for  seed  dissemination.  The  student  is  already  familiar  with  some 


FIG.  283.  Association  of  plants  in  a  forest,  —  blackberries  forming  a  layer 
associated  with  cedar  (Thuja plicata),  Cedar  Mountain,  Idaho 

After  Clements 

of  these  devices  in  the  plants  previously  studied  in  the  field  and 
laboratory.  Thus,  the  wide  distribution  of  willows  and  poplars 
along  the  borders  of  streams  and  on  the  shores  of  lakes  or  ponds 
is  due  to  the  long,  silky  hairs  on  the  seeds,  which  enable  them 
to  migrate  by  means  of  air  currents.  The  rapid  migration  and 
wide  distribution  of  such  composites  as  the  dandelion  and  yarrow 
is  due  to  the  very  effective  parachute  of  hairs  which  serves  as 
a  flying  apparatus  for  the  fruits  of  these  plants.  In  the  case  of 
fruits  with  wings,  like  the  maples  and  pines,  or  of  the  heavy 


438  GENERAL  BOTANY 

fruits,  like  the  nuts  of  the  hickory  and  oak,  the  devices  are  less 
effective  and  serve  for  local  rather  than  for  wide  seed  distribu- 
tion. The  seeds  of  edible  fruits,  like  berries,  apples,  and  cherries, 
are  also  very  widely  disseminated  by  birds  and  other  animals, 
which  cast  the  seeds  with  their  excrements  in  regions  far  re- 
moved from  the  home  of  the  mother  plant.  Almost  innumerable 


FIG.  284.   Invasion  of  a  grass,  Agropyron,  into  bare  sand  by  groups,  Mount 
Garfield,  Pikes  Peak,  Colorado 

After  Clements 

examples  might  be  added  of  other  devices  by  which  the  mobile 
seeds,  fruits,  and  other  reproductive  parts  of  plants  migrate  and 
invade  new  regions. 

The  immediate  effect  of  such  migrations  and  invasions  (Figs.  284 
and  285)  in  the  formation  of  new  plant  associations  is  most  easily 
observed  where  tracts  of  land  occur  which  are  devoid  of  vegeta- 
tion. Such  land  surfaces  may  exist  in  gardens,  lawns,  and  fields, 
or  they  may  be  the  result  of  fire,  flood,  or  other  destructive 


PLANT  ASSOCIATIONS 


439 


agencies.  In  all  such  cases  of  denuded  land  areas  the  first  inva- 
ders have  a  free  field,  without  competition  on  the  part  of  other 
plants,  which  is  a  very  important  factor  in  the  success  of  invaders 
into  old  plant  communities.  The  main  restrictions  on  the  occu- 
pation of  these  naked  surfaces  are  those  imposed  by  the  nature 
of  the  invading  plants  themselves  and  by  the  environmental  con- 
ditions obtaining  in  the  invaded  area.  Thus,  strictly  water-loving 


FIG.  285.   Invasion  of  Pinus  ponderosa  into  plains  grassland, 
Black  Forest,  Colorado 

After  Clements 

plants  will  not  thrive  on  an  upland  tract,  and  plants  accustomed 
to  medium,  or  mesophytic,  conditions  will  not  grow  or  thrive  in  a 
desert  soil.  In  other  words,  the  invading  species  must  be  more  or 
less  closely  adapted  to  the  soil  and  climate  of  the  invaded  region 
in  order  to  survive  and  become  a  permanent  member  of  a  new 
plant  community  or  association.  The  first  invaders  are  usually 
annuals  or  biennials  furnished  with  mobile  seeds,  fruits,  or 
vegetative  parts.  These  are  in  turn  succeeded  by  hardy  peren- 
nials like  the  grasses,  which  may  establish  a  permanent  plant 


440  GENERAL  BOTANY 

association.  In  other  cases,  as  we  shall  learn  later,  the  herbaceous 
plant  association  may  be  replaced  in  time  by  a  shrub  or  a  forest 
plant  association.  Many  plants,  also,  when  once  established  in  a 
given  locality  spread  by  vegetative  means  and  gradually  drive 
out  other  competitors.  This  is  notably  true  of  plants  with  run- 
ners, stolons,  rhizomes,  and  other  underground  parts,  already  dis- 
cussed under  vegetative  reproduction  in  the  first  pa-rt  of  jthe 


FIG.  286.   The  pond  lily,  an  aquatic  with  floating  leaves  and  submerged  stems 

text.  In  the  end  a  new  plant  association  will  be  formed,  composed 
of  plants  adapted  to  the  conditions  of  soil  and  climate  which 
obtain  in  the  given  region.  The  above  sketch  of  the  main  factors 
involved  in  the  making  of  a  new  plant  association  on  a  denuded 
tract  is  probably  a  fairly  accurate  picture  of  the  manner  in  which 
the  existing  plant  associations  which  constitute  the  earth's  vege- 
tation have  arisen. 

Succession.  The  term  succession  involves  the  idea  of  replac- 
ing the  plants  comprising  a  given  association  by  a  new  plant 
population  which  invades  and  finally  occupies  the  ground  for- 
merly held  by  the  old  association.  This  conception  can  be  most 


PLANT  ASSOCIATIONS 


441 


easily  explained  by  a  concrete  illustration  of  succession  often 
exhibited  in  the  history  of  a  pond  or  lake  which  becomes  grad- 
ually filled  with  soil  and  decaying  vegetation.  The  plants 
comprising  the  first  association  in  such  a  habitat  are  wholly 
hydrophytic  and  consist  of  free  masses  of  algse  and  of  shore 
plants  such  as  flags  (7m),  bulrushes  (Scirpus),  and  arrowhead 


:« 


-V--X" 


v 


FIG.  287.   Zonation  of  grass  (Deschampsia),  bulrushes  (Scirpus),  and 
pines  (Pinus  ponderosa) 

After  Clements 

(Sagittaricf).  As  the  pond  or  lake  becomes  shallower  by  the 
accumulation  of  vegetable  remains  and  the  iiiwash  of  soil  the 
shore  plants  encroach  more  and  more  upon  the  water  area,  fol- 
lowed by  grasses  and  sedges  which  convert  the  old  shore  line 
into  a  marsh  or  bog.  This  marsh  or  bog  may  form  a  permanent 
plant  association  for  a  considerable  period  or  it  may  be  converted 
by  willows,  alders,  and  similar  shrubs  into  a  thicket,  and  then  by 
poplars,  ash,  maples,  and  oaks  into  a  typical  mesophytic  forest. 


•5cm.- 


JSOOft, 


FIG.  288.   Diagram  illustrating  zonation  and  succession  around  a  pond 

I,  pond  (a,  floating  pipewort  (Eriocaulori)  •  b,  deepest  portion  of  pond ;   c,  rushes 

(Juncus)  forming  an  association) ;  II,  bog  zone ;  ///,  swamp  thicket  zone ;  IV,  sand 

pit  and  incomplete  xerophytic  zone ;    V,  dry  meadow  zone ;  VI,  dry  woodland  zone 

(d,  birch  woodland).  From  Bergen  and  Davis's  "  Principles  of  Botany  " 


PLANT  ASSOCIATIONS 


443 


The  length  of  time  required  for  one  type  of  association  to  suc- 
ceed and  supplant  another  in  such  an  instance  as  that  just 
described,  as  well  as  the  composition  and  nature  of  the  plant 
populations  which  follow  one  another  in  any  given  succession,  will 
of  course  vary  in  different  cases,  but  the  general  facts  regarding 
succession  will  hold  for  all  similar  habitats.  Another  familiar 
illustration  of  plant  succession  is  often  observed  in.  forests  where 
fires  destroy  the  trees  over  wide  tracts.  The  first  invaders  are 


FIG.  289.   A  hillside  once  forested,  but  now  bare  and  eroded 
Photograph  by  United  States  Forest  Service 

.* 

here,  as  in  most  other  instances,  herbaceous  plants,  including  the 
fire  weeds  (Epilobium  and  Lrechtites),  which  are  able  to  live  and 
thrive  in  the  burned-over  area.  The  herbaceous  plants  are  in  time 
succeeded  by  aspens  and  conifers  in  some  regions,  or  by  forests  of 
deciduous  hardwood  trees  in  others.  Similar  processes  may  be 
observed  along  the  shores  of  most  lakes  and  streams  where  new 
land  is  formed  by  changing  water  levels  or  in  cultivated  fields 
and  gardens  where  land  is  plowed  and  allowed  to  lie  fallow  long 
enough  for  the  first  occupants  to  be  routed  by  later  competitors. 


444  GENERAL  BOTANY 

The  arrangement  of  plants  in  regular  zones,  termed  zona- 
tion  (Figs.  287  and  288),  in  regions  where  one  association  is 
being  replaced  by  another,  is  often  very  evident,  particularly 
on  the  borders  of  streams,  ponds,  and  lakes.  In  some  such 
instances  the  zones  are  very  distinctly  marked,  while  in  others 
they  merge  gradually  into  one  another,  creating  tension  lines 
of  great  competition  where  two  kinds  of  vegetation  struggle 
for  supremacy. 

General  instability  of  vegetation.  The  changes  noted  above  in 
new  plant  associations,  due  to  migration,  invasion,  and  competi- 
tion, are  far  more  general  in  their  nature  than  is  usually  supposed. 
The  great  numbers  of  seeds  formed  by  each  plant  species  in 
nature,  and  the  admirable  devices  developed  for  their  dissemi- 
nation, result  in  a  wide  annual  sowing  of  the  seeds  of  new  species 
in  every  old  plant  association.  If  the  young  plants  which  spring 
from  these  seeds  are  better  adapted  to  the  environment  in  which 
they  chance  to  spring  up  than  the  existing  species,  the  new- 
comers will  gradually  drive  out  the  older  inhabitants  and  become 
a  new  element  in  the  population  of  the  old  association.  Familiar 
instances  of  such  changes  are  seen 'in  the  invasion  of  a  lawn  by 
dandelions,  and  of  roadsides  and  cultivated  fields  by  weeds. 
Old  associations  are  thus  constantly  changing  in  the  kinds  of 
species  constituting  them.  Another  p'otent  cause  for  far-reaching 
changes  in  existing  vegetation  is  the  gradual  change  going  on  in 
the  surface  features  of  the  earth,  caused  by  elevations  and  sub- 
sidences of  the  earth's  crust  and  by  water  erosion.  Geologists 
tell  us  that  the  continents  are  slowly  but  surely  being  leveled 
off  by  the  wearing  down  of  the  hills  and  by  the  deposit  of  the 
eroded  soil  in  the  valleys  by  water  action  (Fig.  289).  By  these 
processes  hills  and  slopes  are  denuded,  new  and  barren  cliffs  are 
formed,  and  existing  plant  populations  are  covered  and  destroyed 
in  the  valleys  and  on  flood  plains.  These  constant  readjustments 
initiate  new  plant  associations,  which  form  the  first  of  a  series  of 
plant  populations,  followed  by  successions  such  as  those  described 
above.  What  is  known  as  vegetation  is  therefore  in  constant 
flux  and  change,  although  the  individual  plants  which  comprise 
it  are  themselves  immobile. 


PLANT  ASSOCIATIONS  445 


SUMMARY 

The  vegetation  of  the  earth's  surface  is  not  homogeneous  but  is 
composed  of  units  of  varying  composition  which  are  termed  associa- 
tions. These  units  of  vegetation  have  apparently  originated,  as  new 
associations  arise  to-day,  in  regions  where  the  land  is  partially  or 
wholly  deprived  of  its  vegetation  by  natural  or  artificial  means. 
Some  of  the  main  factors  concerned  in  the  origin  and  development 
of  a  new  plant  association  are  the  migration  of  plants  by  means  of 
reproductive  parts,  the  adaptation  of  plants  of  different  kinds  to 
different  environmental  conditions,  and  competition  between  the 
individuals  of  a  growing  plant  population.  Plant  associations  have 
also  been  found  to  be  constantly  changing,  owing  to  competition  and 
to  changes  in  habitats  due  to  natural  and  artificial  causes.  The  gen- 
eral instability  of  existing  vegetation  is  the  natural  outcome  of  these 
various  factors,  which  are  in  continual  operation  over  large  areas  of 
the  earth's  surface. 


INDEX 


(References  to  illustrations  are  indicated  by  asterisks  accompanying  page  numbers) 


Absorption,  mechanism  of,  137 

Absorption  by  roots,  139 

Acaulescent,  353 

Accessory  buds,  356 

Acer  saccharum,  88*,  390*,  391* 

Aceraceae,  390,  393 

Achene,  364,  365,  412* 

AchiUea,  flower  of,  411*,  412* 

Achillea  (yarrow),  411* 

Acorn,  385* 

Acuminate  leaf  apex,  355* 

Acute  leaf  apex,  355* 

Adiantum,  embryo  of,  309* 

Adiantum,  gametophyte  of,  309* 

Adiantum    (maidenhair    fern),    297*, 

299*,  300*,  301* 
Adjustments,  summary  of,  44 
Adjustments  of  American  elm,  43* 
Adjustments  of  bean  leaves,  28*,  29* 
Adjustments  of  caladium,  38* 
Adjustments  of  clover,  39*,  40* 
Adjustments  of  common  plants,  37 
Adjustments  of  corn  roots,  32*,  33*, 

34* 

Adjustments  of  dandelion,  41* 
Adjustments  to  environment,  23-45 
Adjustments  of  garden  pea,  24* 
Adjustments  of  nasturtium,  26* 
Adjustments  of  plant  body,  14 
Adventitious  buds,  356* 
Adventitious  roots,  357 
^Eciospores  (secidiospores),  274,  275* 
Aerial  roots,  357 
Aerobic,  249 

Aggregate  fruit,  364*,  365,  405* 
Air  spaces  of  leaf,  115 
Alder,  stem  structure  of,  91*,  92*,  93* 
Alfalfa,  roots  of,  112 
Algae,  219-241 

Algae,  life  histories  of,  228,  229 
Alismaceae,  428-430 
Alnits,  stem  structure  of  92*,  93* 
Alnus  mollis,  stem  sections  of,  91* 
Alternation  of  generations,  292*,  311* 
Amanita  muscaria,  267 
Ambrosia,  19 


American  crab,  408* 

Anaerobic,  249 

Anaphase,  75*,  77 

Anatomy,  213 

Anatropous  ovule,  363 

Anatropous  ovule  of  Ins,  347* 

Anemophilous,  361 

Angiosperms,  337 

Angiosperms,  trees  of,  380 

Animate  environment,  7 

Annual  bean  plant,  physiology  of,  127* 

Annual  bean  plant,  seasonal  history  of, 

126-130,  129* 

Annual  bean  plant,  summary,  130 
Annual  ring,  85* 

Annulus  of  fern  sporangium,  306* 
Annulus  of  mushroom,  268*,  269 
Anther,  162* 

Anther,  structure  of,  343* 
Anther  and  sporophylls,  341* 
Antheridium,  288,  289 
Antheridium  of  Ricciocarpus,  231* 
Anthracnose,  282 
Antipodals,  164,  165* 
Antitoxins,  256 
Apogeotropic,  20*,  30 
Apophototropic,  30 
Apotropic,  30 

Apple,  flower  and  fruit  of,  409 
Apple  tree,  body  plan  of,  15*,  16* 
Aquilegia  (columbine),  399* 
Araceae,  425,  426* 
Archegonium,  289* 
Archegonium  of  Ricciocarpus,  289* 
Arisaema,  426* 
Ascent  of  water,  144,  146 
Ascent  of  water,  path  of,  141* 
Asci  of  lichen,  278*,  280 
Asdepias,  leaf  structure  of,  113*,  115* 
Ascophyllum,  habit  of,  236* 
Aspergillus,  266* 
Aspidium,  fern,  307* 
Assimilation,  125 
Assimilation  of  bean  plant,  129 
Associations,  origin  of,  436 
Associations,  plant,  435,  445 


447 


448 


GENEKAL  BOTANY 


Avena  saliva,  flower  of,  433*,  434* 

Axil  of  leaf,  17* 

Axillary  branch,  17     . 

Axillary  bud,  356 

Axis  of  bud,  68* 

Axis  of  strobili,  339* 

Azalea,  flower  of,  359* 

Bacteria,  250,  258 

Bacteria,  cell  division  in,  252* 

Bacteria,  colonies  of,  257* 

Bacteria,  forms  of,  251* 

Bacteria,  spore  formation  in,  253* 

Bacteria,  spore  germination  in,  254* 

Bark,  84,  85*,  86*,  90,  101* 

Bark,  formation  of,  102 

Bark  in  Salvia,  105 

Basidia  of  mushroom,  269*,  270 

Bean,  growth  of,  67* 

Bean,  sensitive  roots  of,  34,  36 

Bean  plant,  seasonal  life  of,  126, 127*, 
129* 

Bean  pulvini,  15*,  16* 

Bean  roots,  sensitiveness  of,  36 

Bean  seedlings,  4 

Beer  making,  250 

Befry,  364*,  365,  421,  422 

Bidens  (Spanish  needles),  413 

Biennial  plant,  seasonal  life  of ,130,131* 

Biology,  historical  sketch  of,  212 

Biology,  plant,  3 

Biology,  subdivisions  of,  216 

Bird's-eye  maple,  88*,  90,  392 

Black  cherry,  407* 

Black  raspberry,  vegetative  reproduc- 
tion of,  155* 

Blackberry,  flower  and  fruit  of,  405* 

Blackberry  association,  437* 

Blackberry  hybrids,  183 

Body  plan,  summary  of,  23* 

Body  plan  of  apple,  15* 

Body  plan  of  buckwheat,  17 

Body  plan  of  elm,  21*,  22* 

Body  plan  of  herbaceous  plants,  18* 

Body  plan  of  lilac,  15* 

Body  plan  of  pine,  20* 

Body  plan  of  plants,  15*,  16*,  17* 

Bordered  pits,  327,  328 

Botany,  historical  sketch  of,  212 

Bracken  fern,  anatomy  of,  303*,  304*, 
305* 

Bracket  fungi,  272*,  273 

Brassica,  flowers  of,  400* 

Bread  making,  249 

Breeding,  plant  and  animal,  216 

Brown,  Robert,  55 

Brussels  sprouts,  401 


Bryophyta,  287-297 

Buckwheat,  body  plan  of,  17* 

Buckwheat,  growth  of,  60* 

Bud,  lateral,  15* 

Bud  growth,  68*  70*,71* 

Bud  scales,  68* 

Bud  structure,  68*,  70* 

Budding,  245 

Bud-scale  scars,  69* 

Buds,  accessory,  356* 

Buds,  adventitious,  356 

Buds,  flower,  356 

Buds,  kinds  of,  356 

Buds,  lateral,  356 

Buds,  leaf,  356 

Buds,  mixed,  356 

Buds,  supernumerary,  356 

Buds,  terminal,  356 

Bulb,  hyacinth,  418* 

Bulb,  tulip,  157* 

Bulbs,  354 

Bulbs,  culture  of,  420 

Bulbs,  development  of,  422* 

Bundle  scars,  84* 

Burbank  and  plant  breeding,  182,  183 

Buttercup,  398* 

Cabbage,  401 
Cacti,  ornamental,  148* 
Caladium,  adjustments  of,  37,  38* 
Cattha  palustris,  339*,  397* 
Calyptra  of  moss,  295* 
Calyx,  161,  169* 
Cambium  in  alder,  92*,  93* 
Cambium  in  lilac,  84,  85 
Cambium  in  oak,  86 
Cambium  of  roots,  111 
Cambium  in  Salvia,  106* 
Cambium  layer,  95,  96*,  98 
Camptosorus,  vegetative  reproduction 

of,  158* 

Campylotropous  ovule,  363 
Campylotropous  ovule  of  Capsella,  345 
Canal  cells  of  archegonium,  289* 
Capillitium  of  puffball,  271* 
Capsella  (shepherd's  purse),  344,  345*, 

346 

Capsule,  295* 

Capsule  (compound  ovary),  364*,  365 
Capsule  of  moss,  294,  295* 
Carduus  (thistle),  413* 
Carpel,  169* 
Carpels,  363* 
Carrot,  root  of,  356* 
Caryopsis,  365,  434 
Catalytic  agent,  248 
Caulescent,  353 


INDEX 


449 


Celery,  cells  of,  46* 

Cell,  historical  sketch  of,  54,  59 

Cell,  minute  structure  of,  72* 

Cell,  naming  of,  54 

Cell,  summary  of  historical  sketch  of, 

59 

Cell  and  plant  development,  57* 
Cell  division,  73,  75*,  80 
Cell  parts,  functions  of,  51,  52 
Cell  parts,  summary  of,  52,  53 
Cell  plate,  79 
Cell  sap,  52 
Cell  theory,  55 
Cell  wall,  46*,  47*,  48,  54 
Cells,  parts  of,  46,  51 
Cells,  thick-walled,  of  celery,  46* 
Cells  with  plastids,  50* 
Cells  of  root  tips,  47* 
Cellular  structure,  45,  53 
Centgener  plots,  205* 
Cherry,  black,  407* 
Cherry,  flower  and  fruit  of,  409* 
Chlamydomonas,  222* 
Chlorophyll,  50 
Chloroplastids,  49,  50*,  115 
Chromatin,  73 
Chromoplastids,  49,  50* 
Chromosome  reduction,  311 
Chromosomes,  74,  75*,  76 
Chromosomes,  reduction  and  division 

of,  80 
Chromosomes  and  reproduction,  81*, 

160* 

Cineraria,  body  plan  of,  18* 
Circinate  vernation,  298*,  299 
Citranges,  183 
Classification,  213 
Claytonia,  fern,  298* 
Climate  and  water  supply,  367 
Close-fertilization  defined,  174 
Close-pollination  defined,  167,  174 
Clover,  movements  of,  37,  39*,  40 
Clover,  pollination  of,  172,  173 
Clover,  pulvini  of,  39* 
Clover,  seasonal  life  of,  130,  131* 
Cohesion  theory,  145 
Collateral  vascular  bundle,  301 
Collecting  hairs  of  locust,  171 
Color  of  flowers,  362 
Columbine,  399* 
Columella,  262* 
Commelinaceae,  417 
Commercial  relations  of  plants,  12 
Complete  flower,  360 
Compositae,  410*-413 
Compound  pistils,  363* 
Concentric  vascular  bundle,  301* 


Conceptacles,  237*,  238* 
Conduction  in  stems,  99,  100 
Cones  of  long-leaf  pine,  379* 
Cones  of  spruce,  331*,  332*-,  333*,  374* 
Conifer  ales,  327 
Conjugate  nucleus,  228,  335 
Contrasting  characters,  188,  189* 
Convolvulus,  leaf  structure  of,  149* 
Coprinus  comatus,  number  of  spores 

in,  268-271 

Cork  bark,  85*,  86*,  101* 
Cork  cambium,  102 
Cork  layer,  91,  92 
Corm  of  gladiolus,  157* 
Corms,  354 
Corn,  breeding  of,  178*,  179*,  193*, 

194*,  195* 

Corn,  hybridization  of,  178*,  179* 
Corn,  inflorescence  and  flower,  193*, 

433* 

Corn,  plant  of,  193* 
Corn,  root  system  of,  112 
Corn,  stem  structure  of,  109* 
Corn  breeding,  193*,  194*,  195* 
Corn  kernels,  oil  and  protein  of,  195* 
Corn  roots,  growth  of,  61* 
Corolla,  161 
Cortex  in  roots,  110* 
Cortex  in  Salvia,  105,  106* 
Cortex  of  stems,  92* 
Corymb,  358* 
Cotyledon  of  pea,-  24* 
Couch  grass,  431* — 
Crab  apple,  American,  408 
Crenate  leaf  margin,  355* 
Cross-fertilization  defined,  174 
Crossing  and  hybridizing,  174 
Cross-pollination,  167,  174 
Cruciferae,  400*,  401 
Curly  grain  of  maple,  88*,  90 
Cycad,  321,  322* 
Cycadales,  321,  325 
Cyclic  flowers,  358* 
Cyclic- leaf  arrangement,  15*,  16* 
Cypripedium,  427* 
Cypripedium  acaule,  427*,  428* 
Cypripedium  pubescens,  427* 
Cytology,  59 
Cytoplasm,  47*,  52* 
Cytoplasmic  sac,  47*,  50*,  53 
Czapek,  experiment  with  roots,  36 

Dandelion,  flower  and  fruit,  412*. 
Dandelion,  habit  and  flowers,  410* 
Dandelion,  responses  to  stimuli,  40, 

41*,  42 
Darwin,  Charles,  31,  207* 


450 


GENERAL  BOTANY 


Darwin,  experiments  in  crossing,  175- 
180 

Darwin,  experiments  with  roots,  34* 

Decay,  255 

Deciduous  trees,  380 

Dehiscence  of  anther,  162* 

Dehiscent  fruits,  364*,  365 

Dentate  leaf  margin,  355* 

Descriptive  terms,  353 

DeVries,  Hugo,  201* 

DeVries,  mutation  theory  of,  199 

Diageotropic,  20*,  30 

Diaphototropic,  30 

Diatropic,  30 

Dichogamy,  361,  362* 

Dichotomous,  300 

Dichotomous  branching,  235 

Dichotomous  venation,  300 

Dicotyledons,  flower  morphology  of, 
339* 

Dicotyledons,  herbaceous  and  woody, 
396 

Dicotyledons,  megasporangia,  spores, 
and  embryo  of,  349* 

Dicotyledons,  megaspore  and  embryo 
of,  345* 

Dicotyledons,  microspore  of,  348* 

Dicotyledons,  morphology  of,  337,  349 

Dicotyledons,  sporophylls  and  sporan- 
gia, 341* 

Dicotyledons,  stem  structure,  105,  108 

Dicotyledons,  structure  of  Qalvia  stem, 
106* 

Dicotyledons,  summary  of  stem  struc- 
ture, 108 

Dicotyledons  and  monocotyledons, 
413,  414*,  415 

Diffuse-porous  wood,  382* 

Digestion,  124,  125 

Digestion  in  the  bean,  126,  127* 

Dimorphic  flowers,  362* 

Dicscious,  226,  237 

Dioecious  moss,  294* 

Dioecious  poplars,  389* 

Dioecious  willows,  388* 

Dioon  edule,  cycad,  322* 

Disease  caused  by  bacteria,  256 

Disease  caused  by  fungi,  281-285 

Dominance,  Mendelian,  184*,  185 

Dotted  ducts,  94 

Double  fertilization,  346,  347* 

Drupe,  364*,  365,  409*  . 

Dry  fruits,  364*,  365 

Ducts,  dotted,  93*,  94 

Ducts,  primitive,  of  fern,  305* 

Ducts,  water,  85,  93* 

Dwarf  shoots,  380 


East  and  corn  crossing,  178*,  179 

Ecology,  146-153,  435-445 

Ecology  defined,  214 

Egg,  165* 

Egg  apparatus,  164 

Elaeagnus,  hairs  of,  149* 

Elaters,  312*,  313 

Elm,  flowers  of,  395* 

Elm,  slippery,  395 

Elm,  white,  392*,  393* 

Elms,  394* 

Elodea,  49,  152 

Elongation  zone  of  root,  63*,  64,  65* 

Emargiriate  leaf  apex,  355* 

Embryo  of  Capsella,  345* 

Embryo  of  mandrake,  165*,  166* 

Embryo  sac,  164,  165* 

Embryology,  57,  214 

Endodermis  of  fern  rhizome,  305* 

Endodermis  of  roots,  110* 

Endoenzyme,  247 

Endosperm,  165,*  166 

Endothia  parasitica,  281 

Entomophilous,  361 

Environment,  3 

Environment,  adjustments  to,  14,  23 

Environment,  animate,  7 

Environment  of  animals,  11 

Epicotyl  of  pea,  24 

Epidermis  of  leaf,  113*,  114,  115* 

Epidermis  of  root,  110* 

Epidermis  of  Salvia,  106* 

Epidermis  of  stem,  84,  85*,  91 

Epigynous  flower  of  appl"e,  409* 

Epigynous  flower  of  yarrow,  411* 

Epigynous  flowers,  358*,  359 

Equisetales,  311,  315 

Equisetum,  life  history  of,  315* 

Equisetum  arvense,  312* 

Erect  tree  type,  19,  20* 

Erosion,  443* 

Erythronium,  421* 

Erythronium,    bulb    development   of, 

422* 

Evolution,  211,  215,  218 
Exoenzyme,  247 
Exoskeleton  of  stem,  300 

Fagopyrum  (buckwheat),  17* 
False  whorls,  19,  20* 
Fermentation,  246 
Ferments,  digestive,  248 
Ferments,  energy-forming,  248 
Fertilization,  57*,  160 
Fertilization,  double,  347* 
Fibers,  central  spindle,  77 
Fibers,  phloem,  94* 


INDEX 


451 


Fibers,  traction,  77 

Fibers,  wood,  95*,  96 

Fibrous  roots,  357* 

Filament  of  anther,  162 

Fiiicales,  298-311 

Filicales,  anatomy  of,  301*,  303*,  304*, 

305* 

Filicales,  gametophyte,  308* 
Filicales,   gametophyte   and   embryo, 

309* 

Flagella,  222* 
Fleshy  fruits,  364*,  365 
Floods,  forest  control  of,  368 
Floral  plans,  361* 
Flower,  morphology  of,  339* 
Flower,  parts  of,  161,  162*,  163 
Flower  buds,  356 
Flowers,  clustered,  357* 
Flowers,  solitary,  357* 
Follicle,  364*,  365 
Food  cycle,  organic,  11* 
Food  relations  of  plants  and  animals,  9 
Food  storage,  98 
Forces  of  environment,  3 
Forest,  central  hardwood,  383 
Forest  control  of  rainfall,  368 
Forest  products,  370 
Forest  trees,  groups  of,  371 
Forests,  national,  366,  368* 
Form  of  herbaceous  plants,  18* 
Form  of  plant  body,  14 
Form  of  trees,  19*,  20*,  21*,  22,  43* 
Fragaria  (strawberry),  403*,  404* 
Fruit  of  mandrake,  166* 
Fruits,    classified,  365,  366 
Fruits,  kinds  of,  364*,  366 
Fucus  vesiculosus,  235,  241 
Fucus  vesiculosus,  habit  of,  236* 
Fucus  vesiculosus,  life  history  of,  240, 

241 
Fucus  vesiculosus,  reproduction,  237*, 

238*,  239*,  240* 
Funaria,  293-297 
Funaria,  gametophyte  and  sex  organs, 

294* 

Funaria,  sporophyte,  295* 
Fungi,  242-285 
Fungi,  classification  of,  243 
Fungi  and  disease,  281,  285 
Funiculus  of  ovule,  162*,  165*,  169* 

Gametangium  in  Spirogyra,  227 
Gametes,  57*,  159,  160*,  163* 
Gametes,  female,  164 
Gametes,  male,  164* 
Gametic  purity,  187 
Gametogenesis,  163 


Gametophyte  in  algas  (Spirogyra),  226 
Gametophyte  in    angiosperms,   343*, 

348*,  349* 
Gametophyte     in     bryophytes,     288, 

289* 
Gametophyte   in  gymnosperms,   322, 

323*,  334*,  335* 
Gametophyte  in  pteridophytes,  308*, 

309*,  319* 

Gap,  leaf,  of  ferns,  300*,  301* 
Gap,  leaf,  of  pines,  327*,  329* 
Garden  pea,  Darwin's  experiments 

with,  177 

Generative  cell  of  cycad,  323* 
Generative  cell  of  pollen,  164* 
Generative  cell  of  spruce,  334* 
Geotropic,  30 
Geum  triflorum,  410 
Gladiolus,  corms  of,  157* 
Gleba  of  puffball,  271*,  272 
Glumes  of  oat  flower,  434 
Grain  of  wood,  87*,  88,  90 
Gramineae,  430,  435 
Grass,  couch,  431* 
Grasses,  430 
Gravity,  root  response  to,  33*,  34*, 

36* 

Gravity  sense,  31 
Growth,  67,  72 
Growth,  summary  of,  69,  72 
Growth  of  buckwheat,  60 
Growth  of  corn  root,  61* 
Growth  of  herbaceous  stems,  67* 
Growth  of  leaves,  66* 
Growth  of  lilac  bud.  68* 
Growth  of  root-tip,  61*,  64,  65*,  67* 
Growth  of  roots  and  stems,  70* 
Growth  of  root-tip  cells,  63* 
Guard  cells,  113*,  114,  115* 
Gymnosperms,  321-336 
Gymnosperms,  Coniferales,  325 
Gymnosperms,  Cycadales,  321 
Gymnosperms,  forest  trees,  371-380 

Habit,  353 
Habitat,  353 
Hairs,  protective,  149* 
Halberd-shaped  leaf,  355* 
Halophytic  association,  435 
Hanstein,  57 

Hardwood  trees,  380,  381*,  382* 
Head,  358* 

Heart-shaped  leaf,  355* 
Heartwood,  86 
Hepaticae,  28Y-293 

Herbaceous  plants,    body    plan  and 
form  of,  18* 


452 


GENERAL  BOTANY 


Herbaceous  stem,  growth  of,  67*,  70* 

Herbaceous  stems,  dicotyledons,  105- 
108 

Herbaceous  stems,  monocotyledons, 
108,  109* 

Herbaceous  stems,  structure,  106* 

Herbaceous  stems,  summary  of  struc- 
ture, 108 

Heterosporous,  314,  316,  318* 

Heterostylous,  362* 

Histology,  214 

Holly,  wood  of,  382* 

Homogamous,  362 

Homosporous,  313,  316 

Hooke,  Robert,  54 

Hyacinth,  plant  of,  418* 

Hybridization  of  corn,  178*,  179* 

Hybridization  and  new  varieties,  180 

Hybridization  of  plums,  182* 

Hybridization  of  wheat,  180*,  181* 

Hydrophytes,  152*,  153 

Hydrophytic  association,  435,  440* 

Hymenium  of  mushroom,  269*,  270 

Hypha,  258 

Hypocotyl  of  pea,  24* 

Hypogynous  flowers,  358*,  359* 

Imperfect  flowers,  360* 
Inanimate  environment,  3 
Inanimate  environment,  forces  of,  3 
Inanimate  environment,  materialsof,  5 
Inbreeding  and  crossing,  175 
Inbreeding  denned,  174 
Income  and  outgo  of  animals,  9 
Income  and  outgo  of  plants,  8 
Incomplete  flowers,  360* 
Indehiscent  fruits,  365,  368* 
Industrial  biology,  215 
Industrial  relations  of  plants,  12 
Inflorescence,   nature   and   kinds  of, 

357*,  358* 

Inflorescence  and  pollination,  107, 108 
Inorganic  foods,  9 
Integuments,  162*,  164,  165* 
Internode,  4*,  15*,  16*,  17* 
Invasion,  ecological,  438*,  439*,  463 
Invasion  by  fungi,  283 
Invertase,  248 
Ipomcea    (sweet    potato),    vegetative 

reproduction  of,  158* 
Iridaceae,  423-424* 
Iris,  ovule  and  pollen  tube  of,  347* 
Iris  versicolor,  424* 
Irregular  flowers,  360 

Jacket  cells,  334* 
Jack-in-the-pulpit,  426* 


Keel  petals,  168,  169* 
Knight,  experiment  of,  32* 
Knight,  Thomas  Andrew,  31 

Labellum  of  Cypripedium,  428* 

LadyVslipper,  427* 

Lamellse  of  mushroom,  268*,  269* 

Lanceolate  leaf,  355* 

Lateral  bud,  356 

Leaf,  epidermis  of,  113* 

Leaf,  section  of,  115* 

Leaf  arrangement,  15*,  16* 

Leaf  blade,  4*,  15*,  17* 

Leaf  gaps,  300*,  301*,  327*,  329* 

Leaf  petiole,  4*,  15*,  17* 

Leaf  scars,  69*,  84* 

Leaf  structure,  113,  116 

Leaf  trace,  300*,  301*,  302*,  327*  329* 

Leaves,  cyclic  and  spiral  arrange- 
ments, 16* 

Leaves,  form,  venation,  margins,  and 
apex,  355* 

Legume,  171*,  364*,  365 

Leguminosce,  39 

Lemma  of  oat  flower,  434* 

Leucoplastids,  49,  50* 

Lichens,  277,  278*,  279*,  280 

Life  histories  of  algae,  228-229 

Life  history  of  Fucus,  240,  241 

Life  history  of  GEdogonium,  235 

Life  history  of  Vaucheria,  233 

Lilac,  body  plan  of,  15*,  16* 

Lilac,  bud  growth,  68*,  71* 

Lilac,  leaf  growth,  66 

Lilac,  stem  structure,  85* 

Lilac  twigs,  69*,  84* 

Liliaceae,  418,  423 

Lily,  anther  and  pollen  of,  343* 

Lily,  double  fertilization  in,  347* 

Lily,  microsporangium  and  spores,  343* 

Lily,  pond  association,  440 

Linear  leaf,  355* 

Lip  cells  of  Sporangia,  306* 

Lobed  leaf  margin,  355* 

Locust,  inflorescence  and  flower  of, 
171* 

Locust,  pollination  of,  170,  173 

Locust,  seasonal  life  of,  133* 

Lodicules  of  oat  flower,  434* 

Lupine,  Darwin's  experiments  with, 
177 

Lycopodiales,  315,  317 

Lycopodium,  316* 

Maidenhair  fern,  297*,  299* 
Malt,  250 
Maltase,  248 


INDEX 


453 


Mains  (apple),  408 
Mandrake,  flower  of,  161,  162* 
Mandrake,  fruit  of,  166 
Mandrake,  ovules  and  seed  of,  165* 
Mandrake,  pollen  and  ovary  of,  164* 
Mandrake,  seedlings  and  fruit  of ,  166* 
Map  of  forest  areas,  368* 
Maple,  390*,  391* 
Maple,  bird's-eye,  392 
Maple,  curly,  392 
Maple,  flowers  of,  390* 
Marigold,  marsh,  307*,  339* 
Mass  culture,  205 
Materials  of  environment,  5 
Mechanism  of  movements,  28 
Megasporangium,  317,  318* 
Megaspore,  165*,  317,  318*,  319* 
Megaspores,  homology  of,  349* 
Megasporophyll,  317,  318* 
Megasporophylls,  morphology  of,  341* 
Mendel,  Gregor,  185 
Mendelian  ratio,  186 
Mendel's  laws,  184*,  188*,  189*,  190* 
Mendel's  principles  of  heredity,  184* 
Meristem,  61,  62*,  63* 
Mesophyll  of  leaf,  114,  115* 
Mesophytes,  146,  147* 
Mesophytic  association,  435,  436* 
Mesophytic  vegetation,  147* 
Metaphase,  75*,  77 
Metaplasmic  bodies,  52 
Micropyle,  162*,  164,  165* 
Microsporangium,  317,  318* 
Microspore,  317,  318*,  319* 
Microspores     of     Selaginella,  spruce, 

and  mandrake,  348 
Microsporophyll,  317,  318* 
Microsporophylls,  morphology  of,  341* 
Midvein  of  leaf,  113*,  115* 
Migration  and  invasion,  438 
Migration  of  plants,  436 
Milkweed,  leaf  structure  of,  113*-116* 
Mimosa  (sensitive  plant),  58. 
Mirabilis,  crossing  of,  186* 
Mitosis,  73-80 
Mitosis,  function  of,  73 
Mitosis,  phases  of,  75*-80 
Mitosis,  process  of,  74 
Mold,  Aspergillus,  266* 
Mold,  black,  259*,  260,  261* 
Mold,  blue,  264*,  265* 
Molds,  258-267 
Monocotyledons,  416 
Monocotyledons,  structure  of,  108, 109* 
Monocotyledons     and     dicotyledons, 

413,  414*,  415 
Monoecious,  360,  385* 


Morning-glory,  Darwin's  experiments 

with,  176 
Morphology,  213 
Moss,  292* 
Mosses,  293,  296 

Mosses,  Funaria,  293,  294*,  295* 
Motor  organs,  pulvini,  29*,  39* 
Mucronate  leaf  point,  355* 
Musci,  293,  296 
Mushrooms,  267*-271 
Mushrooms,  Amanita,  267* 
Mushrooms,  spore  formation  of,  268*, 

269* 

Mustard  family,  400 
Mutations,  199 
Mutations  in    (Enothera,   200*,  202*, 

203* 
Mycelium,  258 

Nageli,  55,  56 

Nasturtium,  phototropic  response  of, 

26* 

Neck-canal  cells,  289* 
Nectar  of  flowers,  362 
Node,  4*,  15*,  16*,  17* 
Nuclear  division,  73,  75*,  80 
Nucleolus,  47*,  53 
Nucleus,  47*,  53,  55 
Nursery  of  yellow  pine,  377* 
Nut,  364*,  365,  385* 
Nutrition,  117 
Nutrition  of  animal,  9* 
Nutrition  of  greerrplant,  8* 
Nyctitropic  (sleep)  position,  38,  40 

Oak,  sections  of,  89* 

Oak,  white,  383*,  384*,  385* 

Oak  stem,  structure  of,  86* 

Oat,    inflorescence    and    flowers    of. 

433*,  434* 

Oblanceolate  leaf,  355* 
Oblong  leaf,  355* 
Obovate  leaf,  355* 
Obtuse  leaf  apex,  355* 
Odor  of  flowers,  362 
(Edogonium,  233,  235 
(Edogonium,     asexual     reproduction, 

235* 

(Edogonium,  sexual  reproduction,  234* 
(Enothera,  seedlings  of,  202 
(Enothera  gigas,  203* 
(Enothera    lamarckiana,    199*,    200*, 

203* 

(Enothera,  lata  and  nanella,  200 
Oogonium,  231* 
Oogonium  in  Vaucheria,  230 
Operculum  of  moss,  295* 


454 


GENERAL  BOTANY 


Orbicular  leaf,  355* 

Orchidaceae,  427*,  428 

Organic  food  cycle,  11*,  285* 

Organic  foods,  9 

Ornithogalum,  419* 

Orthotropous  ovule,  363 

Osmosis,  137,  139* 

Osmosis,  experiment  in,  138* 

Osmotic  pressure,  138 

Outgo  of  animal,  9* 

Outgo  of  plant,  8* 

Oval  leaf,  355* 

Ovary,  simple  and  compound,  363* 

Oviate  leaf,  355* 

Ovules,  164,  165* 

Ovules,  campylotropous,  363 

Ovules,  kinds  of,  363 

Ovules,  orthotropous,  363 

Palet  of  oat  flower,  434* 

Palisade  tissue  of  leaf,  113*,  115* 

Palmate  venation,  354,  355* 

Panicle,  358* 

Panicle  of  oat,  433* 

Papilionaceous     flowers,    pollination 

of,  168 

Pappus  of  dandelion,  412* 
Parallel  venation,  354,  355 
Paraphyses  of  moss,  293,  294* 
Paraphyses  of  mushroom,  269*,  270 
Parasites,  242 

Parenchyma,  palisade,  113*,  115* 
Parenchyma,  phloem,  92*,  93* 
Parenchyma,  spongy,  of  leaf,  115* 
Parenchyma,  wood,  93*,  94 
Parenchyma  of  cortex,  92 
Parmelia,  279* 
Pasteurization,  255 
Path  of  water,  141* 
Pathology,  plant  and  animal,  215 
Pea,  crossing  of,  1 78 
Pea,  flower  of,  169* 
Pea,  tropisms  of,  24* 
Peas  and  Mendel's  laws.  184*    188*, 

189*,  190* 

Pedigree  culture,  205 
Peduncle,  161,  357* 
Penicillium,  264*,  265* 
Perennial,  seasonal  life  of,  133* 
Perfect  flowers,  360* 
Perianth  of  flower,  161 
Pericycle,  111 

Pericycle  of  fern  rhizome,  305* 
Pericycle  of  roots,  111 
Peridium  of  puffball,  271 
Perigynous  flowers,  358*,  409* 
Peristome  of  moss,  295* 


Permanent  zone  of  root,  62,  63* 

Petiole,  15*,  17* 

Phloem,  84,  85*,  92* 

Phloem,  elements  of,  96*,  97 

Phloem,  fibers  of,  92* 

Phloem,  parenchyma  of,  92* 

Photosynthesis,  117-120,  127* 

Photosynthesis  and  respiration,  123 

Phototropic  response,  30 

Phototropism  of  nasturtium,  26* 

Physcia  stellaris,  278* 

Physiology,  58,  214 

Physiology  of  bean  plant,  127* 

Picea  (spruce),  forest  of,  325,  326* 

Picea  (spruce),  gametophytes  of,  334* 

Picea  (spruce),  ovule  and  seed  of,  335 

Picea  (spruce),  sporangia  and  spores 
of,  333 

Picea  (spruce),  strobili  of,  331*,  332* 

Picea  (spruce),  wood  structure  of, 
327*,  328*  329* 

Pileus  of  mushroom,  268*,  269* 

Finales,  325,  327 

Pine,  cones  of  long-leaf,  379* 

Pine,  section  of,  378* 

Pine  nursery,  377* 

Pines,  erect  type,  19,  20* 

Pines,  forest  trees,  378-380 

Pinnate  venation,  300,  354,  355 

Pinnule,  300 

Pinus  palustris,  cones  of,  379*,  380 

Pistil,  162*,  169* 

Pistils,  compound,  363* 

Pistils,  kinds  of,  363* 

Pistils,  simple,  363* 

Pisum  sativum,  crossed  and  self-fer- 
tilized plants  of,  178 

Pith,  85 

Pits,  bordered,  327*,  328* 

Placenta,  162*,  163 

Placentae,  kinds  of,  363* 

Plankton,  219 

Plant  associations,  435,  445 

Plant  associations,  kinds  of,  435 

Plant  associations,  origin  of,  436 

Plant  body,  plan  of,  14 

Plant  disease,  281 

Plantain,  vegetative  reproduction,  156* 

Plants,  industrial  and  commercial  re- 
lations of,  12 

Plants,  relations  to  animals,  8,  9 

Plastids,  49,  50*,  51 

Pleurotus  ulmarius,  272 

Plums,  hybridization  of,  182* 

Plumule  of  pea,  24* 

Polar  nuclei,  164,  165*,  347* 

Pollen,  163,  164* 


INDEX 


455 


Pollen  chamber  of  Zamia,  323* 

Pollen  formation,  343* 

Pollen  germination,  343* 

Pollen  tube,  164*,  323*,  325,  334*, 
347* 

Pollen  tube  of  Iris,  424* 

Pollination,  166-173 

Pollination,  kinds  of,  167 

Pollination  and  inflorescence,  167 

Pollination  in  locust,  170,  171* 

Pollination  in  papilionaceous  flowers, 
168,  169* 

Pollination  in  red  clover,  172,  173 

Polyporus  squamosus,  273* 

Pome,  364*,  365,  409* 

Pomelo,  183 

Pond  lilies,  152* 

Pond-lily  association,  440* 

Pontederia,  429* 

Pontederiaceae,  428 

Poplars,  389* 

Populus  deltoides,  389 

Potamogeton,  152 

Potato,  vegetative  reproduction  of, 
150*,  156 

Potato  (sweet),  vegetative  reproduc- 
tion of,  158* 

Primary  root,  356 

Primrose,  Lamarck's,  199* 

Proembryo,  335 

Progeotropic,  20*,  30 

Pronuclei,  160,  228 

Prop  roots,  193*,  357 

Prophase,  74,  75* 

Prophototropic,  30 

Protandrons  flowers,  362 

Protococus,  220,  221* 

Protogynous,  362,  404* 

Protonema  of  moss,  296* 

Protoplasm,  47* 

Protoplasm,  naming  of,  55 

Protoplast,  47,*  52 

Protropic,  30 

Prunus  (cherry),  407 

Prunus  serotina,  407* 

Pteris  aquilina,  302-305 

Pteris  aquilina,  anatomy  of,  303*,  304*, 
305* 

Ptomaine  poisoning,  258 

Ptomaines,  257 

Puccinia  graminis,  274,  275* 

Puffballs,  270*,  271*,  272 

Pulvini,  28*,  29*,  37,  39* 

Pulvini  of  bean  leaf,  28*,  29* 

Pulvini  of  clover,  39* 

Ptirkinge,  55 

Putrefaction,  255 


Pyrenoids  of  Spirogyra,  224* 
Pyrus  coronaria,  408* 

Quarter-sawed  oak,  87*,  88*,  89* 
Quercus  alba,  383*-386 
Quercus  alba,  flowers  and  fruit  of,  385* 
Quercus  rubra,  wood  of,  89* 

Raceme,  357* 

Rachis  of  fern  leaf,  299* 

Radish,  401 

Rainfall,  forest  control  of,  368 

R amentum,  299 

Ranunculaceae,  397*,  399 

Raphe,  364 

Raspberries,  405 

Rays,  wood,  86*,  87*,  92*,  93*,  94 

Receptacle  of  flower,  161,  339*,  340 

Receptacle  of  Fucus,  237* 

Red  clover,  pollination  of,  172,  173 

Red  clover,  root  system  of,  111 

Red  oak,  cuts  of,  89* 

Reduction  division,  80*,  81*,  82 

Regular  flowers,  360 

Reproduction,  sexual,  159,  160*,  161 

Reproduction,  vegetative,  154 

Resin  canals,  326 

Respiration,  120-124,  127* 

Respiration  and  photosynthesis,  123 

Rhizoids  of  Ricciocarpus,  287,  289* 

Rhizome,  299,*  353,  354* 

Rhizophore,  317* 

Rhizopus,  259*,  240*,  261*,  262*,  263* 

Ricciocarpus,  288-293 

Ricciocarpus,  life  history  of,  291* 

Ricciocarpus,  reproductive  organs  'of, 

289* 
Ricciocarpus,  sporophyte   and   spores 

of,  290* 

Ring-porous  wood,  381* 
Root,  functions  of,  112 
Root,  hairs  of,  110* 
Root  cap,  61*-62*,  63* 
Root  hairs  and  absorption,  140*,  141* 
Root  pressure,  138* 
Root  system  of  clover,  111* 
Root  system  of  corn,  112*,  193*,  357* 
Root  tip,  cell  structure  of,  62*,  63* 
Roots,  anatomy  of,  110* 
Roots,  arrangement  of,  18,  19 
Roots,  fleshy,  fibrous,  357 
Roots,  growth  of,  61*,  63*,  65*,  70* 
Roots,  primary  and  secondary,  4*,  17*, 

193* 

Roots,  prop,  of  corn,  193*,  357 
Roots,  sensitive  zone  of,  34*,  36* 
Roots,  soil,  water,  aerial,  357 


456 


GENERAL  BOTANY 


Roots,  tap,  4*,  356* 

Kootstock  of  Solomon's  seal,  354 

Rosa  (rose),  406 

Rosaceae,  403-410 

Rose,  flower  structure  of,  405* 

Rubus  (raspberries  and  blackberries), 

405 

Runners,  354 
Runners  of  Rhizopus,  261 
Runners  of  strawberry,  155 
Rusts,  274-275* 

Sachs,  experiment  by,  33* 
Sachs,  Julius  von,  31,  59 
Sagittaria,  429* 
Salicaceae,  387-390 
Salix  discolor,  388 
Salma,  bark  of,  105 
Salvia,  cortex  of,  105 
Salma,  stem  structure  of,  105, 106*,  107 
Salma,  summary  of  structure  of,  108 
Salma,  vascular  cylinder  of,  105,  106* 
Samara,  364*,  365,  390* 
Saprolegnia,  282 
Saprophytes,  242 
Sapwood,  86 
Sassafras,  wood  of,  381* 
Scale,  ovuliferous,  332 
Scales  of  bud,  68* 
Scape  of  dandelion,  40,  41*,  42 
Scars,  bud-scale,  69*,  84* 
Schizomycetes,  250 
Schleiden,  M.  J.,  55,  56 
Schultze,  Max,  56 
Schwann,  55 
Seasonal  life,  125 
Seasonal  life,  summary  of,  135 
Seasonal  life  of  Adiantum,  300 
Seasonal  life  of  annual,  126-130 
Seasonal  life  of  bean,  129* 
Seasonal  life  of  biennial,  130, 131*,  132 
Seasonal  life  of  perennial,  133*,  134 
Secondary  roots,  356* 
Sedges,  430 
Seed,  165*,  166,  169* 
Seed  of  mandrake,  165 
Seed  of  mandrake  and  spruce,  349 
Seed  of  pea,  24* 
Seed  of  spruce,  335* 
Seed  production  in  forest,  373 
Seedling  of  bean,  4*,  129* 
Seedling  of  mandrake,  166 
Seedling  of  pea,  24* 
Seedling  of  spruce,  335* 
Selaginella,  317*-320 
Selaginella,  gametophytes  and  embryo 
of,  319* 


Selaginella,  life  history  of,  320* 
Selaginella,  sporangia  and  spores  of, 

318* 

Selection,  plant  improvement  by,  192 
Selection  in  corn,  194* 
Selection    of    fluctuating    variations, 

208-210 

Selection  of  mutations,  210 
Selection  in  tobacco,  198* 
Self-fertilization,  174 
Self-pollination,  167 
Sepals,  161 

Serrate  leaf  margin,  355* 
Seta  of  moss,  293,  295* 
Sexual  reproduction,  159 
Shasta  daisy,  182,  183 
Shull,  George  H.,  and  corn  crossing,  179 
Shull  and  seedlings  of  CEnothera  la- 

marckiana,  202* 
Sieve  plates,  92,  93*,  305* 
Sieve  tubes,  92,  93*,  305* 
Silique,  363*,  365,  400* 
Silver  grain  of  wood,  88-90 
Simple  pistils,  363* 
Sinuate  leaf  margin,  355* 
'Skeletal  tissue,  86*,  301* 
Skeleton  of  plants,  49 
Smilacina,  420 
Smilacina,  root  structure,  62 
Smilacina  stellata,  421* 
Smuts,  276,  277* 
Soil,  composition  of,  141* 
Soil  and  root  hairs,  140*,  141* 
Solitary  flowers,  357* 
Solute,  137* 
Solvent,  138 

Sori,  274,  275*,  306*,  307* 
Sorus  of  fern,  306,  307* 
Spadix,  426* 
Spanish  needles,  413 
Spathe,  426* 
Spatulate  leaf,  355* 
Spiderwort  (Tradescantid),  417* 
Spike,  358* 
Spikelet,  433* 
Spindle,  75*,  76 

Spiral,  leaf  arrangement,  15*,  16* 
Spiral  flowers,  358 
Spirogyra,  223-229 
Spirogyra,  'fertilization  discovered  in, 

57 

Spirogyra,  life  history  of,  229 
Spirogyra,  reproduction  in,  227* 
Spirogyra,  structure  of,  224*,  225* 
Spongy  tissue  of  leaf,  115* 
Sporangiophore   of  Equisetum,   312*. 

313* 


INDEX 


457 


Sporangiophore  of  molds,  261* 
Sporangium  of  fern,  306*,  307* 
Sporangium  of  mold,  261*,  262* 
Spore  chambers  of  puffball,  271 
Spore  tetrads,  290*,  314 
Sporidia  of  rust,  275* 
Sporidia  of  smuts,  277 
Sporophore  of  puffball,  271* 
Sporophyte  in  angiosperms,  337 
Sporophyte  in  bryophytes,  290*,  295* 
Sporophyte  in  gymnosperms,  321,  325 
Sporophyte  inpteridophytes,  299, 309*, 

Spreading  tree  type,  21*,  22* 
Spring  wood,  85*,  86*,  87* 
Spruce,  325-337,  326*,  371-378 
Spruce,  anatomy  of,  327*,  328*,  329* 

330 

Spruce,  commercial  importance  of,  375 
Spruce,  embryo  and  seed  of,  335* 
Spruce,  gametophytes  in,  334*,  335* 
Spruce,  life  history  of,  336 
Spruce,  regrowth  of,  376*,  378 
Spruce,  second  growth  of,  375* 
Spruce,  seed  production  in,  373,  374* 
Spruce,  strobili  of,  331*,  332*,  333* 
Spruce,  tolerance  of,  372* 
Spruce  and  balsam,  373* 
Spur  shoots  of  apple,  409 
Stalk  cell,  323* 
Stamens,  162* 

Standard  of  pea  flower,  168,  169* 
Starch  grains,  50* 
Starch  sheath,  224* 
Stem  structure  of  alder,  91* 
Stem  structure  of  corn,  109* 
Stem  structure  of  dicotyledons,  105, 

106* 

Stem  structure  of  lilac,  85* 
Stem    structure   of    monocotyledons, 

108,  109* 

Stem  structure  of  oak,  86* 
Stem  structure  of  Salvia,  106* 
Stems,  function  of  tissues,  98 
Stems,  functions  of,  83 
Stems,  growth  of,  67*,  70*,  71* 
Stems,  kinds  of,  353,  354 
Stems,  multiple,  of  plantain,  156* 
Stems,    vegetative    reproduction    of, 

154-157 

Sterigmata  of  mushroom,  269*,  270 
Sterilization,  254 
Stigma,  163,  164*,  169* 
Stimuli,  internal,  29 
Stimuli,  kinds  of,  30 
Stimulus,  27 
Stipe  of  mushroom,  268* 


Stolon,  354 

Stolon  of  Rhizopus,  261* 

Stoma  of  fern  sporangium,  306* 

Stomata,  113*,  114,  115* 

Storage  in  stems,  98,  99 

Strasburger,  Eduard,  56,  59 

Strawberry,  flower  and  fruit  of,  404* 

Strawberry,   vegetative  reproduction 
of,  155* 

Strawberry  plants,  403* 

Strobili,  313 

Structure  of  stems,  roots,  leaves,  83, 
117 

Stylar  brush,  412* 

Style,  163,  169 

Subhymenium  of  mushroom,  269* 

Succession,  440,  442* 

Sugar  cane,  430* 

Sugar  maples,  grove  of,  391* 

Summaries  : 

Relations   of  plants  to  environ- 
ment, 13 
Body  plan,  23 

Adjustments  by  tropisms,  44 
The  parts  of  the  cell,  52,  53 
The  cell  and  the  cell  theory,  59 
Growth  and  cell  division,  69-72 
Structure     and     physiology     of 

trees,  100-103 

Structure  of  herbaceous  stems,  108 
Seasonal  life  of  bean,  130 
Seasonal  life,  135,  136 
Variation  antt  selection,  206-208 
Evolution,  211 
Biological  sciences,  216 
Bacteria,  258 
Fern  anatomy,  305 
Spruce  anatomy,  330 
Angiosperms,  348,  349*,  350 
Plant  associations,  445 

Summer  wood,  85*,  86* 

Supernumerary  buds,  356 

Suspensor,  319*,  335* 

Sweet    potato,    vegetative    reproduc- 
tion in,  158* 

Sycamore,  wood  of,  382 

Symbiosis  of  lichen,  280* 

Synergidse,  164,  347* 

Tangelo,  183 

Tangerine,  183 

Taproot,  4*,  17*,  356* 

Taraxacum  (dandelion),  410* 

Taxonomy,  212 

Teliospores  (teleutospores),  275*,  276 

Telophase,  75*,  78 

Terminal  bud,  356 


458 


GENERAL  BOTANY 


Tetrads,  81,  290* 

Tetrads  in  lily,  343* 

Tetrads  in  Ricciocarpus,  290* 

Thallophyta,  287 

Thallus,  287 

Thistle,  413 

Timothy,  variation  in,  204* 

Tobacco,  Connecticut  broad-leaf,  197* 

Tobacco,  Uncle  Sam  Sumatra,  198* 

Tolerance  in  forest  trees,  372,  373* 

Tracheae,  93 

Tracheids,  304,  327,  328* 

Tradescantia  (spiderwort),  417* 

Trama,  270 

Transpiration,  127*,  142-146 

Transpiration,  control  of,  143 

Trees,  erect  type,  19,  20* 

Trees,  form  and  development  of,  19 

Trees,   growth  and  bark  formation, 

102 

Trees,  hardwood,  380 
Trees,  importance  and  use  of,  366 
Trees,   long  and  transverse  sections 

of,  101* 

Trees,  longevity  of,  103 
Trees,  physiology  of,  104 
Trees,  poplar,  389* 
Trees,  responses  to  stimuli,  43* 
Trees,  shrubs,  and  forests,  366 
Trees,  spreading  type,  21*,  22,  43* 
Trees,   structure   and  physiology  of, 

100-105 

Trees,  sugar  maple,  390*,  391* 
Trees,  white  elm,  393*-395 
Trees,  white  oak,  383*,  387 
Trees,  willow,  387*-389 
Trees  and  forests,  366-380 
Trifolium  pratense,  37,  39* 
Trifolium  pratense,  pollination  of,  172 

173 

Trimorphic  flowers,  362 
Tropisms,  23,  27 
Tropophytes,  150 
Tube  cell,  334* 
Tube  nucleus,  164*,  347* 
Tubers,  354 

Tubers  of  potato,  156*,  334 
Turnip,  401 
Twig,  lilac,  69*,  84* 

Ulmaceae,  393-396 

Ulmus  Americana,  392*,  395* 

Ulmusfulva,  393,  335* 

Ulmus  racemosa,  393 

Umbel,  358* 

Undulate  leaf  margin,  355* 


Urediniospores     (uredospores),     274, 

275* 
Ustilago  zeae,  276,  277* 

Vacuoles,  47*,  52 

Variation,  fluctuating,  197 

Variation,  summary,  206 

Variation  in  apples,  192* 

Variation  in  corn,  194* 

Variation  and  selection,  192 

Variation  in  timothy,  204 

Variation  in  tobacco,  197 

Vascular  cylinder  in  Salvia,  105,  106* 

Vascular  plants,  299* 

Vascular  system  of  fern,  301*,  302 

Vaucheria,  230-233 

Vaucheria,  asexual   reproduction   in, 

233* 
Vaucheria,  sexual  reproduction  in,  230, 

231*,  232* 

Vegetation,  instability  of,  444 
Vegetative  reproduction,  154-159 
Vegetative  reproduction  in  fern,  158* 
Vegetative  reproduction  in  gladiolus, 

157* 
Vegetative    reproduction    by  leaves, 

157,  158* 
Vegetative  reproduction  in  plantain, 

156* 
Vegetative    reproduction    in    potato, 

156* 
Vegetative  reproduction  in  raspberry, 

155* 

Vegetative  reproduction  by  roots,  158* 
Vegetative  reproduction  by  stems,  154, 

155*,  156*,  157* 

Vegetative    reproduction    in    straw- 
berry, 155* 
Vegetative     reproduction     in     sweet 

potato,  158* 

Vegetative  reproduction  in  tulip,  157* 
Veil,  268 

Veins  of  leaf,  113*,  115*,  116 
Venation  of  leaves,  355* 
Venter  of  archegonium,  289* 
Ventral-canal  cells,  289* 
Viciafaba,  34 

Violapinnata,  flower  structure  of,  402* 
Violaceae,  401*-403 
Violet  flowers,  401*,  402* 
VonMohl,  55,  56 

Walking  leaf  fern,  158 

Water,  ecological  relations  of  plants 

to,  146-153 
Water,  relation  of  plants  to,  137-153 


INDEX 


459 


Water  ascent,  144-146 

Water  ascent,  path  of,  141* 

Water  ascent,  rate  of,  145 

Water  ducts,  93* 

Wheat,  rust  of,  274,  275* 

White  oak,  383*~386 

White  oak,  inflorescence  and  flowers 

of,  385* 

White  oak  timber,  384* 
White  sweet  clover,  seasonal  life  of, 

130,  131* 
Willow,  387,  388* 
Willow,  reproduction  of,  388* 
Wing  petals,  168,  169* 
Wood,  destruction  by  fungi,  273* 
Wood,  diffuse-porous,  382* 
Wood,  ring-porous,  381* 
Wood,  sections  of,  87*,  88*,  89* 
Wood,  spring  and  summer,  85*,  86 
Wood  of  alder,  91*,  92*,  93* 
Wood  of  lilac,  85* 
Wood  of  maple,  88* 
Wood  of  oak,  86*,  101* 
Wood  of  red  oak,  89* 
Wood  cylinder,  84 
Wood  fibers,  93*,  94 
Wood  parenchyma,  92*,  93*,  94 
Wood  rays,  87*,  94 

Xerophytes,  148* 


Xerophytes,  leaf  structures  of,  149* 
Xerophytic  association,  435,  438* 
Xylem,  92*,  93* 
Xylem  elements,  96*,  97,  98 

Yarrow,  inflorescence  and  flowers  of, 

411*,  412* 

Yeast,  spore  formation  in,  247 
Yeast  cells,  245* 
Yeasts,  234-250 
Yeasts,  budding  of,  245*,  246* 
Yeasts,  top  and  bottom,  244* 
Yeasts,  wild  and  cultivated,  244* 
Yeasts,  wine  and  beer,  244* 

Zamia,  321-325 

Zamia,  life  history  of,  324* 

Zamia,  strobilus,  sporangia,  and  game- 

tophytes,  323 

Zea  mays,  root  system  of,  112*,  193* 
Zea  mays,  stem  structure  of,  109* 
Zonation,  441*,  442*,  444 
Zoosporangium  of  VaucJieria,  233* 
Zoospore  of  CEdogonium,  235* 
Zoospore  of  Vaucheria,  232,  233* 
Zoospores  of  Chlamydomonas,  222 
Zoospores  of  OEdogonium,  235 
Zoospores  of  Vaucheria,  233 
Zygote,  57,  159,  160* 
Zymase,  246 


ANNOUNCEMENTS 


TEXTBOOKS  IN  BIOLOGY 

FOR  HIGH   SCHOOLS  AND  COLLEGES 
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Bergen  :    Botanies  (For  list  see  High-School  and  College  Catalogue) 

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With  or  without  Key  and  Flora 
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Gruenberg :  Elementary  Biology 
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Meier:   Herbarium  and  Plant  Description 
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Penhallow :  Manual  of  North  American  Gymnosperms 
Roth:  First  Book  of  Forestry 

BACTERIOLOGY 

Conn :  Bacteria,  Yeasts,  and  Molds  in  the  Home  (Rev.  Ed.) 
Moore :  Laboratory  Directions  for  Beginners  in  Bacteriology 
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Linville  and  Kelly :   Laboratory  and  Field  Work  in  Zoology 

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Meier:  Animal  Study 

Pratt:   Course  in  Invertebrate  Zoology  (Rev.  Ed.) 

Pratt :  Course  in  Vertebrate  Zoology 

Sanderson  and  Jackson  :  Elementary  Entomology 

PHYSIOLOGY 

Blaisdell :  Life  and  Health 

Blaisdell:   Practical  Physiology 

Brown :   Physiology  for  the  Laboratory 

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Hough  and  Sedgwick :  The  Human  Mechanism  (Rev.  Ed.) 

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